Land Imprinting Methods

Prepared for Discovery Park, Safford, Arizona,
by Ted St. John, Ph.D.

THE NEED FOR IMPRINTING

Seed is far less expensive than container planting in most revegetation situations, and is the only available option on the great majority of very large-scale projects. Since simple broadcasting of seed is unreliable, ecologists and rangeland scientists have devised seed application methods that greatly improve germination and seedling establishment. Among the most successful of these methods is land imprinting.

Land imprinting is the formation by mechanical means of short, smooth-walled V-shaped furrows in the soil surface. Imprinting depends upon a heavy tractor-drawn roller, armed with metal teeth, to form the furrows. The implement used for this purpose, the land imprinter, is able to not only shape the soil, but also apply seed, and even place beneficial mycorrhizal fungi beneath the soil surface.

The imprinting pattern provides improved exchange of water and air, erosion protection, and good contact between seeds and soil. The imprints collect rainwater and permit it to infiltrate, even if soil surface conditions make infiltration a very slow process. Without imprints, rainwater runs off and collects in the lowest parts of the terrain, often causing erosion. Only the soil under the depressions becomes charged with moisture, and only those areas can meet the needs of germinating seeds. Land imprinting forces infiltration over the entire soil area, permitting much more uniform establishment of plants.

A very important result of imprinting is improved exchange of air between the soil and the atmosphere. Good soil aeration encourages the soil organisms that make a natural plant community possible.

The imprinting pattern provides soil heterogeneity, required for successful germination and early establishment of plants. The impressions provide "seedling cradles," protective depressions similar to natural "safe sites" that offer moisture, shade, cover from small herbivores, and other plant requirements. Loose soil, blowing organic matter, and other material collects in the depressions and increases over time the effectiveness of the seedling cradles.

Imprinting is generally of most value in arid environments, although it has been used in a variety of climates.

Dr. Bob Dixon, the inventor of imprinting, has summarized the factors that make the most difference in imprinting of dry land sites: good seeds, good imprints, and good rains.

Good imprints: The most important single factor in a successful imprinting project is properly formed imprints. The capture of rainfall, the infiltration of soil moisture, the germination of seeds, and the trapping of resources in the depressions all depend upon the quality of the imprints.

Good seeds: The choice of seeds for imprinting is critical. Good seeds not only germinate well, but also include the right plant species. The characteristics of the selected plant species must be compatible with the project and must give a high probability of success in project conditions, factors which are discussed further in a later section. It is crucial that the imprinting seed mix include some aggressive, fast-growing species to serve as nurse plants.

Good rains: Soil moisture is fundamental to project success. Although we have little control over rainfall, we can time the imprinting operation for maximum likelihood of success. In the southwestern United States, November and December are usually the best months for imprinting. Fortunately many of the seeds used in imprinting projects have a long lifespan, and can last until a better rain year if necessary. Dr. Bob Dixon reports that some projects have appeared as long as five years after imprinting.

Land Imprinting Compared To Related Methods

Land imprinting was developed for improvement of the degraded pasturelands and abandoned farmlands that came to occupy much of Arizona and the southwest during the twentieth century. Imprinting was not the first, nor was it the most widely used of the approaches that were developed for dry seeding of these lands. Instead it was developed to specifically address the problems of poor water infiltration and poor soil aeration caused by sealing of the soil surface. Imprinting has been successful in large part because it does a better job than other methods of meeting the infiltration problem, while providing surface heterogeneity and other benefits that were the strengths of the earlier methods. Because a land imprinter is not always available, and because an individual project might in some cases favor an alternative method, some of the other soil preparation methods will be briefly reviewed here.

Broadcasting: Simple broadcasting of seeds is generally regarded as wasteful and ineffective, but is sometimes used when better methods are unavailable. Broadcasting is the method used in aerial seeding of burns, where the land areas are vast and even the fastest land-based methods are too expensive. These efforts generally result in well-fed birds and rodents, and some germination of the weediest plant species in the seed mix.

Scarification: A rough land surface is far superior to a smooth or hard surface for seed establishment, and broadcasting may be considerably improved by roughening the land surface with a tractor-drawn harrow prior to broadcasting. Other implements used for the purpose include other types of plows. After broadcasting, the harrow may be used again to partially burry the seeds, giving some protection from seed-eating animals. Scarification may not be practical on ground with grasses and other small plants that are considered desirable.

Chaining: A special case of scarification is the application of seeds at the same time that undesired vegetation is removed by chaining. Chaining consists of dragging an anchor chain between two tractors, pulling out shrubs and small trees while roughening the soil. Broadcast seeds are partially buried or fall into the crevices in the roughened ground, providing improved conditions for germination. Chaining is not suitable for very rough or steep terrain, are areas where the existing shrubs and trees are considered a desirable part of the vegetation.

Drilling: One of the most widely used planting methods is the rangeland drill. This tractor-drawn device uses disks to cut a groove in the soil, and then drops seeds into each groove. The machine then covers and tamps the soil over the seeds. Rangeland drilling has given good results over extensive land areas in the southwest, and continues to be a valuable method. It is most suitable for retired farmlands, where the ground is level and smooth, and relatively free of rocks, debris, and existing vegetation. The best seed drills have separate bins for seeds of different sizes, because the dispensing devices often perform poorly where a seed mix includes a range of seed sizes and shapes. The rangeland drill is unsuited for steep or very rough and rocky terrain.

Contour furrowing: A shank attached to a powerful tractor is sometimes used to form a trench that follows the contours of the hillsides. Seeds are then placed into the trench. The trench is able to give protection from direct sunlight and wind, and is thought to place the newly emerging plants within reach of moisture stored in deeper soil.

Pitting: Several kinds of devices, including hand labor, can be used to scoop depressions in the soil, in a method that shares some characteristics with land imprinting. The depressions may be very large and widely spaced, in which case they are best formed with the blade of a bulldozer. Smaller, regularly placed pits may be formed with a series of shanks on the toolbar of a tractor. The toolbar is raised and lowered as the tractor moves, giving a regular pattern that superficially resembles the pattern left by an imprinter. A machine made from a disk type plow may also form regular pits. A section of each disk is removed with a cutting torch, leaving disks that alternately cut and leave high ground as the device is pulled by a tractor. Mechanical pitters are difficult to use on very steep or rocky ground, or in areas where there is valuable preexisting vegetation. However, such sites are suitable for hand pitting. A hoe, shovel, pick, or other hand tools may be used on any terrain that is accessible to workers. In this way pitting has been used to establish vegetation on slopes too steep for any mechanical method other than hydraulic seeding.

Hydraulic seeding: "Hydroseeding" is an expensive method not devised for improvement of large land areas. It has been used in some revegetation work because the equipment is familiar to landscape architects who might design revegetation projects in urban areas. If done well, the method works on slopes that may be too steep even for hand labor.

Livestock: Large animals make depressions in moist soil that collect rainwater and other resources. If seed are present, the hooves of the livestock press the seeds into firm contact with the soil. These "living imprinters" can produce results that are superior to almost any of the mechanized methods. However, it is important that the livestock be removed quickly once their work is done, since the same animals that plant the seeds can quickly overgraze the resulting vegetation. Livestock may be the method of choice on remote sites where cattle, horses, or other large animals are available and can be removed within a few days.

Comparisons of methods: All of the available methods have been used with success in some conditions, and all have failed in other conditions. Certain characteristics of the methods make some more suited for particular jobs.

Broadcasting, scarification, and rangeland drilling allow water to run downhill and collect in gullies or other low spots, leaving most of the soil without stored moisture. Livestock, imprinting, pitting, and contour furrowing trap runoff throughout the land surface, permitting even storage of soil moisture. Unless the land is quite level, these methods have a substantial advantage in the arid west.

Seeds germinate best when in firm contact with the soil. This is always achieved by livestock and by properly executed land imprinting, and is sometimes achieved with rangeland drilling. Pitting and scarification leave most of the seeds in only loose contact with the soil.

In rough, rocky, or steep terrain, the suitable methods usually are limited to broadcasting, hand pitting, hydroseeding, livestock, and certain types of land imprinting. Contour furrowing may apply in some cases. If there is already desirable vegetation on site, the same list applies. If there is a desirable cryptobiotic crust (layer of algae and lower plants on the soil surface), pitting and contour furrowing are not suited since they destroy this valuable layer.

If the soil is badly overgrazed, unvegetated, or freshly graded, mycorrhizal inoculation may be the only way to create a healthy diverse plant community. The inoculum must be placed below ground to be effective. Scarification, chaining, contour furrowing, pitting, and land imprinting may all be used to incorporate mycorrhizal inoculum. However, the only way to produce a continuous below ground network of mycorrhizal fungi, a necessary part of most healthy native ecosystems, is with closely spaced inoculum. Only scarification, land imprinting and very closely spaced pits offer evenly spaced inoculum.

Construction of the Land Imprinter

A number of land imprinters are in use in Arizona and California, but at the time of this writing there is no manufacturer in routine production. Several machine shops have constructed land imprinters, as have individuals with the facilities and skills to cut, handle, and weld large pieces of metal.

Detailed specifications for the construction of a land imprinter, including diagrams and measurements, are available from the Imprinting Foundation, 1231 East Big Rock Road
Tucson, Arizona 85718 (520 297-6165). The specifications have also been posted at www.mycorrhiza.org , where they can be downloaded in PDF format.

Imprinters constructed to date have cost between $6000 and $15,000, depending on the size and complexity, and whether the owner did part of the construction. Several contractors in California and elsewhere own imprinters and will undertake imprinting work within a reasonable distance of their base of operations. Discovery Park has an older style land imprinter, and parts of several others, that are available for loan to qualified agencies and individuals.

The land imprinter includes several subsystems: the frame, the roller, the ballast, and the material delivery mechanisms.

Frame: A substantial metal frame holds the other components in their proper spatial relationships, and provides a means for a tractor to tow the assembly. The tow bar is detachable for transport, and may be attached to either end of the symmetrical frame. By providing the ability to tow the unit in either direction, the operator may distribute wear on both the front and back surfaces of the metal teeth, extending the life of the roller.

Roller: The roller consists of a cylinder 20 to 24 inches in diameter. Larger rollers used in the past proved very difficult to transport. Rollers smaller than 20 inches often slide rather than roll in wet soil, and tend to round the tops of the ridges. The length of the roller is usually limited to eight feet, since longer rollers leave large patches of flat ground when one end rides up on a rock. There is no theoretical limit to the width on level, stone-free ground, but it is generally preferable to gang several rollers rather than construct a single very long roller.

The teeth of the imprinter push soil both forward and backward, into the cavity between the teeth. Most imprinter teeth are 6" or 8" on a side, resulting in troughs 41/4" or 55/8" deep. As with all aspects of equipment design, there are trade-offs with tooth size. Larger teeth may be necessary to cut through accumulated vegetation, or smaller teeth may be needed for higher plant density.

Tooth shape is another variable in imprinter design. Teeth wider than 90 ° have trouble cutting into the soil. Teeth narrower than 90 ° may bury the seed too deep. However, narrower angles are appropriate on steep slopes or in very hard soil. The most current tooth design is a tooth with a 90 ° angle but the legs pressed inward to give a narrow base. This design penetrates the soil well yet maintains the advantages of tooth height.

The apex of the angle may in some cases be offset to the rear, giving a "saw tooth" pattern. Saw tooth patterns work well on steep slopes, but the long side must always be oriented upslope to hold water. Imprinters with saw tooth patterns are not reversible to distribute tooth wear, and have been replaced by symmetrical teeth with narrow bases.

Imprinter teeth are ideally 10" in length, since longer teeth can lead to depressions that accumulate too much water and contribute to erosion. The imprinter should be designed so that the ends of the teeth are separated by two inches, leaving a dam between impressions to prevent movement of water.

The teeth are arranged in rings around the drum. The rings are 12" wide, counting the 10" tooth length and 2" between rings, to give a water-confining wall between imprints. Adjacent rings are offset by one half the width of a tooth, so that the depressions left by the roller are staggered.

Ballast: Ballast tanks are balanced over the roller, to help maintain a center of gravity directly over the axle. They are filled with water and have a provision for easy drainage. They should have sufficient capacity that the total weight over the axle can be varied from 500 to 1000 lb. per foot of roller length. I.e., an imprinter with an eight-foot roller should be adjustable from two to four tons in weight.

Older style imprinters with a very large diameter roller used the roller itself to hold water or a mixture of soil and water. Newer imprinters use tanks both in front of and behind the roller to hold water. Other systems have used boxes full of bricks, or trays that could accommodate large metal castings or old engine blocks. Newer imprinters require less weight than the old style with large rollers. The smaller diameter rollers with staggered rings of teeth mean that fewer teeth contact the ground at any one time. This means that the weight is distributed among fewer teeth, and less weight is required to achieve the same quality of imprint.

Material delivery: Seeds and mycorrhizal inoculum are delivered from bins onto the surface of the roller, where they are pressed into contact with the soil, or incorporated a short distance into the soil. The bins are often fertilizer or seed boxes purchased from agricultural equipment manufacturers. The seed and inoculum boxes may be manufactured according to the construction specifications available from the Imprinting Foundation. The agitator and drive mechanisms in that design are elegant in their simplicity, and have proven more effective and reliable than many of the complex commercial designs.

If mycorrhizal inoculum is fed from a separate supply it may be injected in lines into the ground through hollow shanks, or dropped through tubes behind a series of small ripping teeth or disks. In most cases the inoculum is mixed with the seed. Although some of the inoculum does not get properly incorporated when it is mixed with the seed, the wasted inoculum has turned out to be a fair trade for the extra labor that was required to clean and maintain a separate delivery system.

Other designs: Most of the people who have built their own imprinters have introduced variations on the basic design, either to reduce construction costs or to add what the builders considered to be improvements. In most cases these have not worked out well. Examples include a roller so small that it slides rather than rolls when the ground is wet, a unique tooth design that leaves more flat ground than imprinted area, and a frame of light design that cannot accommodate enough weight to properly imprint hard soils. Other "improvements" have included low-cost bearings on the roller that resulted in immediate breakage, and a drive mechanism for a seed bin agitator that was so complex that the operator spent more time repairing than using it. Most of the mistakes have resulted from a misunderstanding of the reasons for the original design, or a lack of experience in field operation with the stock version. In general, it is better to reserve modifications to the imprinter's design until after one has accumulated some experience with the basic model.

Dr. Bob Dixon has continually improved the imprinter over the years, and the most recent models bear only limited resemblance to the earliest versions. A recent variation includes a design that clamps onto the blade of a bulldozer, with the weight provided by the tractor's hydraulic system rather than ballast. This version can climb steep slopes, limited only by the angle at which the bulldozer can apply downward pressure to the roller. It can also be clamped on a rear toolbar, again with the weight provided by the tractor's hydraulics. This style is lighter than the tow style, and thus less expensive to build and easier to transport. However it does not perform as well on rough ground where the independent swiveling of the tow model is an advantage.

Use of the Land Imprinter

The imprinter is adjusted for conditions at each project, with the loading of the ballast tanks the primary adjustment. Most soil conditions may be met by loading the machine to a total weight between 500 and 1000 lb per foot of roller length. The exceptions are very heavy clay soils, which may be too hard to imprint when dry, and very sandy soils, which do not hold the imprint shape very well.

The desired imprints have smooth walls with the seeds firmly pressed into the soil. The soil surface is occupied fully by depressions, ridges, and walls between the imprints. The bottoms of the depressions and the tops of the ridges are sharp rather than rounded. If the soil is left with a rolling series of indistinct depressions, with the walls consisting of loose clods rather than smooth soil, the soil was too hard or the imprinter was not weighted properly.

The smooth, well-firmed walls of a good imprint leave the seeds firmed into the soil so that capillarity can re-wet the seeds from the moisture that collects at the base of each impression. Enhanced capillary re-wetting accounts for much of the advantage of imprinting over other seeding methods.

If the soil is too hard for good imprints, it needs moisture or mechanical ripping. About one half inch of rain prepares most soils for imprinting. One half inch of water moistens a sand to about a foot and a clay to three or four inches, usually enough for good imprints. Sandy soil may be imprinted immediately after a rain, but clay soil should be allowed to dry a few hours so it will not stick to the roller.

Ripping is the method of last resort because it damages whatever structure the soil may have. Ripping shanks may be spaced twelve to eighteen inches apart, depending upon the configuration of the tool bar. Wide spacing is usually satisfactory in hard soils, which tend to break apart between the ripping shanks. Disking and tilling are even more harmful to soil structure than ripping, and should be avoided.

The imprinter can make organic debris and even living plants into beneficial mulch, but a large accumulation of weeds or brush can prevent successful imprinting. Brittle woody material is less troublesome than tough, ropy weeds. Large imprinting teeth (8x8"), or those with a sharp angle, work best in heavy vegetation. Shrubs and perennial grasses usually recover well after imprinting, allowing the use of imprinting to improve partially degraded native vegetation.

During imprinting the soil must be compacted enough to assure good capillary movement of moisture, but loose enough to allow root growth. It is important not to load the imprinter any more heavily than necessary for the soil conditions.

The plants used in the imprinting seed mix represent the desired flora, but must also include species that have important roles in the process of rehabilitating the land. Roles played by the plant species include soil protection, weed suppression, sheltering of more vulnerable plant species, and building the network of mycorrhizal fungi.

The best "weed beaters" are generally fast growing natives that are good mycorrhizal hosts. In many cases these are short-lived perennial grasses and composites. Most of these plants are intermediate in succession between the pioneer and the late successional (climax) species. These tend to be good at building the network of mycorrhizal fungi in the soil, which protects against erosion, suppresses weeds, and lays the foundation for growth of the final suite of late successional plant species.

The late successional species are often left out of the seed mix. Conditions at the time of seed application are rarely suitable, and these species tend to require pretreatment of seeds for good germination. They will often find their way to the site in later years when the original vegetation becomes inviting to birds and mammals. They may also be planted from containers when conditions become favorable.

Mid- and late-seral species are also included, but will fail without the protective and soil-building actions of the nurse crops. Late-successional species are rarely good competitors against an established weed population. The worse the potential weed problem, the "weedier" the seed mix species must be.

The seeds should represent a diversity of suitable plant species, since higher diversity plant communities better resist destructive forces, and are better habitat for most kinds of wildlife. On large land areas the seeds that make up the bulk of the mix should be relatively inexpensive, should germinate without elaborate pre-treatment, and should stand up well to storage and mechanical handling.

There is no hard rule for determining the amount of seed to apply, but the total amount often lies between 10 and 20 lb. per acre. It is best to determine from the seed supplier or by testing the number of seeds per pound, the fraction of seeds that will germinate, and the proportion of the bulk seed mix that consists of seeds rather than chaff or other materials. This pure, live seed (PLS) count is used to determine the number, rather than the weight of seeds per acre. For land imprinting, about 250 PLS per square yard of ground surface, all species combined, gives a good general guideline.

Seed for use in the imprinter is mixed with wheat bran to prevent sorting by size. Bran is available from feed dealers, and the most desirable type is "red flaky wheat bran". The mixing ratio with seeds, or with seeds and granular mycorrhizal inoculum, is usually 1:1 by volume. If the seed is "trashy," it will be necessary to agitate it vigorously to thoroughly mix the bran and the seed. Any mycorrhizal inoculum should be added after hammer mill treatment, although it can withstand gentler forms of agitation.

If the soil is devoid of native mycorrhizal fungi, as would be the case on eroded, graded, or overgrazed land, the project must be inoculated at planting, or all plant species must be non-hosts. That is, the seed mix must be limited to the species that do not need the mycorrhizal symbiosis. The most widely used examples are members of the genus Atriplex. A few other species can survive without symbionts, and these are generally recognizable as the weediest among the natives.

The regular pattern of depressions left by the land imprinter suggests that the resulting vegetation will resemble an orchard more than a wildland. This turns out not to be the case. Plant species sort themselves by their requirements for such spatially diverse factors as soil chemistry, depth, or texture, position on the slope, and local moisture conditions. Within three to five years the great majority of imprinting sites have looked very much like natural vegetation, with little sign of the original rows, and plant species forming local single species patches.

Other Factors That Influence Success Rate

Weeds: Imprinting often gives native perennials an advantage over weeds, an advantage that depends largely on the quality of the impressions. Proper firming of the soil allows repeated re-wetting of the seeds by capillarity, providing the longer wetting periods required for germination of most natives. The quality of the impressions can thus make the difference between a stand of natives and a weed patch. The deck is further stacked against the weeds if mycorrhizal fungi are present, either as native fungi in the soil or in inoculum applied at the time of imprinting. Mycorrhizal fungi must usually be supplied on graded, overgrazed, or weedy land.

Fertilizer has a very limited place in restoration of large land areas, both because of its cost and because of its stimulatory effect on weeds. Unless the soil is seriously deficient in one or more nutrient elements, avoid fertilization. The standard for deficiency should not be the one applied to agriculture or horticulture, but should be by comparison to undisturbed soil of the same type, supporting healthy native vegetation.

Herbivores: Herbivory is often the single most difficult problem in habitat restoration, especially in the desert. Herbivores have markedly less effect on large than on small plots; thus there is a distinctly better chance of success if the largest possible areas are treated. Small areas are indefensible if herbivores are abundant. While fences and herbivore control may be temptations during the early stages of an imprinting project, they are rarely required on large plots. By the time the vegetation becomes established, the herbivores, predators, and vegetation will have reached an equilibrium that should allow coexistence.

Imprinting at Discovery Park

Discovery Park in Safford, Arizona used land imprinting to treat several tracts of land after removal of saltcedar and other weedy vegetation. Most of the imprinted areas are performing beyond all expectation, with diverse native shrubs and perennial grasses now thriving on land that once seemed destined to host only weeds forever. Discovery Park now has a functional imprinter of an old design, acquired from the US Agricultural Research Service (ARS) from machines built by Dr. Bob Dixon during the 1970s. In addition, Discovery Park has components that could be used to make several more of these old style machines. By agreement with the ARS, this equipment is available for loan to qualified users.

Taking Action

If you are a land manager you may wish to hire, borrow, or construct a land imprinter to improve the land under your care. The information in this document and the sources listed below will help you decide whether imprinting is available and cost effective for your own situation. If imprinting turns out to be impractical, it may be possible to use livestock, some form of pitting, or another method described in this document to improve your land.

If you are not a land manager, you may have influence in land management as a volunteer or a conservation activist. Learn about land imprinting and related methods, and consider whether they meet the needs of projects that concern you. Visit Discovery Park, or learn of other land imprinting sites from the Imprinting Foundation. Study the successes and failures, and consider how to build on the successes.

A great deal of farmland has been retired in Arizona and elsewhere, largely due to changes in water laws and water availability. Most of this land lies unproductive and covered with Russian thistle and other weeds. Its functionality as watershed and wildlife habitat could be greatly improved by properly imprinting it with seeds of native vegetation. Consider how you might take part or organize such efforts in your area. Determine land ownership, cooperative potential of landowners and other interested persons, and the funding or arrangements that would be required to imprint these areas.

Local Resource Conservation Districts, along with the Natural Resource Conservation Service (NRCS), may be able to advise or help find funding for imprinting projects that serve the missions of those organizations. If your local offices are unfamiliar with imprinting, help find information and examples to bring them up to speed.

If you have the land and resources available, carry out a demonstration project so that local landowners and officials can view the results of land imprinting at first hand. Contact the Imprinting Foundation or Discovery Park for help in planning such projects.

Imprinting contractors

Nature's Image
Trabuco Canyon, California
(949) 454-1225

Native Landscapes
Escondido, California
(760) 735-8700

Hydroplant
San Marcos, California
(760) 744-7360

Gary Burchette,
Fallbrook, California
(760) 728-4400