Soil Rehabilitation Guidebook

[Soil Rehabilitation Guidebook of Contents]

Appendix 2. Revegetation techniques3

Grass and legume seeding

For British Columbia coastal and interior conditions, several variations of similar seed mixes have been successful under a wide range of conditions. Tables A2.1 and A2.2 list seed mixes for general use, dry sites, wetlands and ditches, high elevations, and special use on coastal and interior sites.

Use fall (cereal) rye (Secale cereale) as a cover crop for fast, temporary cover, either alone or with a perennial seed mix. It forms a very extensive root system that is excellent for stabilizing soil and restoring structure. It is also a winter-active annual that will give way to native plants over time on most sites. Because it gets quite tall (up to 1 m), it may be undesirable along main roads or in areas of high fire hazard. Fall rye is an inexpensive way to get quick cover while waiting for the proper time to seed permanent cover. For example, fall rye may be seeded in late fall followed by a perennial mix in the spring.

Developing seed mixes

Seed mixes generally include several species to take advantage of different site conditions, growth forms (e.g., sod formers and bunchgrasses), establishment rates, and persistence. Legumes such as clover, trefoil vetch, alfalfa and lupin are included for nitrogen fixation and aesthetics. Because they generally require more moisture than grasses, the component of legumes is reduced in mixes for dry sites. Adding tree seeds to grass/legume mixes is usually unsuccessful. Some guidelines for developing seed mixes appear below, but for problem sites or special situations, consult an erosion control specialist, agrologist or seed dealer.

Ordering seed

When ordering seed:

Application methods

Apply seed by either dry seeding, wet broadcast seeding, or hydroseeding. The soil materials, slope and local climate determine the most suitable method. Figure A2.1 shows recommended application methods for different slope angles.

Figure A2.1. Recommended seeding techniques for different slope angles.

Dry seeding There are four methods of dry seeding: hand broadcast, motor-driven cyclones, seeding with air blowers and seeding by helicopter.

Hand broadcast seeding: Flat or gently sloping areas (<50%) can be seeded by hand. Rotary type, "belly grinder" seeders are inexpensive and simple to use. Seeding 1 ha in an hour or less is possible.

Walking speed, cranking speed and seeder spill rate determine the application rate and area coverage. Calibrate the seeder by testing a known weight of seed on a small patch typical of the area to be seeded. Start with a small seeder orifice (usually scaled with numbers) and walk at a pace that can be sustained for the entire seeding operation. The natural tendency is to seed too heavily. Observe the swath width and plan a route to allow for overlap to ensure complete coverage. Seeding on snow makes it easier to control coverage.

Calibrate and spread seed and fertilizer separately. To prevent corrosion, always rinse the metal seeder parts after applying fertilizer.

Motor-driven cyclones: The speed of broadcast seeding can be improved by using a motorized seeder (e.g., Herd seeder or cyclone seeder). A seeder mounted on equipment used for ripping landings and roads will reduce application costs and ensure that seed is applied to a freshly prepared seedbed. If mounted on a 4-trac, large areas can be covered quickly.

Air blowers: Equipment such as the Fertiblasterδ uses an air compressor to blow seed or fertilizer up to 10 m. This method is best suited for roadsides because the equipment requires vehicle transport. Approximately 2–5 km of road can be seeded per hour using this method. Coated seed is recommended for improved ballistics. It is very important to calibrate the application rate because seed is delivered quickly. Apply fertilizer and seed separately.

Helicopter: Inaccessible areas with gentle to moderate slopes can be dry seeded using a spreader bucket slung from a helicopter. Abandoned roads with pulled-up fills are suitable candidates if they are not too steep and not easily hand seeded.

Wet broadcast seeding

The wet broadcast seeding system mixes dry grass and legume seeds with water, and immediately discharges it on the area to be seeded.

The system is appropriate where dry seeding would otherwise be prescribed, and offers the following advantages over dry seeding: seed is carried further by the water jet; a larger surface area is treated per unit time; better control of seed dispersal is possible; and seed germination is accelerated and enhanced.

This technique is most useful for revegetating landings, skid roads, backspar trails, borrow pits and other disturbed areas with limited access. It is also highly effective in "temporary erosion control" on recently built mainline and secondary forest roads where immediate surface erosion control is demanded immediately after road construction or when full-scale hydroseeding in late season on interior sites is not practical. (In such cases, full-scale hydroseeding should be planned and carried out the next spring.) A word of caution: wet broadcast seeding is not a substitute for hydroseeding, as it does not apply soil binder. It therefore cannot be used on cut slopes steeper than 60%. It can be used on steep fill slopes, however, immediately after road construction, when the surface is rough, the soil medium is still not compacted, and aeration and water percolation are all conducive to seed germination and plant development.

A description of the operating principles and components of a wet broadcast seeding system is provided in Appendix 3.

Hydroseeding

Hydroseeding (hydraulic seeding) is the application of a water slurry of seed, fertilizer and soil binding agent (tackifier), with or without mulch. Use hydroseeding on open slopes greater than 60%, where tacking the seed to the slope is necessary. There are two types of hydroseeding: ground-based and helicopter.

Ground-based: With this type of hydroseeding, truck-mounted equipment is used to apply the slurry on roadsides and accessible landslides. The equipment consists of a mixing tank with mechanical or hydraulic agitation and a volume pump. The equipment can seed up to 50 m. For longer distances or windy conditions, use a fire hose to increase application distance (up to 100 m downhill).

Helicopter: For inaccessible areas, use the truck-mounted mixing tank (4000–8000 L) to fill a spreader bucket (300–400 L/load) slung from a helicopter. For helicopter applications, add a suspension agent, mulch, or both to the slurry to prevent settling during the trip from the staging area to the seeding site. Aerial hydroseeding is suited primarily to landslides or inaccessible road fills.

Appendix 4 gives general mixing and application instructions for ground-based and helicopter hydroseeding.

Seeding rates

Seed weight, seed quality, seedbed characteristics, climate, erosion potential, soil type and application method all affect the appropriate seeding rate for a particular site.

For example, Timothy has 2 800 000 seeds/kg whereas fall rye has about 40 000 seeds/kg. Applying 10 kg/ha of Timothy seed would provide 28 000 000 seeds/ha, or 2800 seeds/m2, whereas applying 10 kg/ha of fall rye would provide
40 seeds/m2.

The presence of weed seed, diseases and the minimum per cent germination allowed for seed is regulated through the Seed Act. This information should be available from the seed dealer. For seeds typically used in forest soil rehabilitation, the germination per cent may decline from over 95 per cent initially to 75 per cent after one year. A quick germination test using a moist cloth in a petri dish is useful for evaluating the quality of seed that has been stored for more than a year.

A well-prepared seedbed provides the best germination potential, and will allow some flexibility in reducing seed rates. Seedbed suitability declines rapidly after rainfall, so application rates should be increased if seeding is delayed until after a rainfall.

For erosion control in British Columbia's wet coastal and moist interior environments, 1500–3000 live pure seeds per square metre is recommended as a guide (40–80 seeds would occupy an area half the size of this page). On dry sites, the recommended rate for drought-tolerant bunch grasses such as crested wheatgrass can be reduced to about half of wet-site rates. Higher rates may be required for rapid establishment of plant roots to prevent resettling of decompacted soils, or to provide fast erosion control.

Examples of seeding rates for a typical seed mix (e.g., seed mix 1 in Table A2.1 with 1 673 000 seeds per kg) are illustrated below:

Dry seeding: 15–30 kg/ha (2500–5000 seeds per square metre)
Wet broadcast seeding: 20–40 kg/ha
Hydroseeding: 50–85 kg/ha

Seeding rates are higher for hydraulic seeding because seed may be damaged during mixing, pumping and application (extra is also added for spillage, inaccurate area estimates, and a "success insurance" given the high cost of application). For similar reasons, but to a lesser degree, the same applies to wet broadcast seeding. Helicopter seeding uses rates at the higher end of the hydroseeding range to ensure adequate cover. Wind drift and spillage during bucket filling and transport can reduce the amount of seed actually applied.

Because seed is a small portion of the total cost of application, using minimal rates is usually not a good way to try to save money.

Binders

Soil binders or tackifiers are made from vegetable gum, beeswax, seaweed extracts or polyurea polymers. They are added to a hydroseeding slurry to stick the seed to the soil during germination. They also provide temporary erosion protection by holding surface soil particles in place.

Application rates depend on site conditions, equipment and specific products (the range is typically 10–50 kg of binder per hectare). Soil binder can be the most expensive part of a hydroseeding slurry, accounting for 40–60% of it. Use higher rates for steeper slopes, moist and wet sites, and aerial seeding. For helicopter seeding, special soil binders (e.g., J-Tack AS) are used to keep solid particles in suspension if the slurry is not agitated in the bucket.

Flat or gentle slopes may not require soil binder. You can reduce binder rates when using wood fibre mulch, provided that the soil erosion hazard is not extreme.

Polymer binders may not be compatible with fertilizers, as excessive agitation can cause foaming. Check the manufacturer's specifications before use.

Relative cost and effectiveness

Table A2.3. Comparison of the relative costs and effectiveness of various seeding techniques

  Effectivenessa   Relative
Treatment Before After   costb

Hand broadcasting: seed, fertilizer

0 1–3   1
Wet broadcast seedingb

0 2–4   1
Hydroseeding: seed, fertilizer, binderc

1 4–6   1.5
Hydroseeding: seed, fertilizer, binder,
wood fibre mulch at 1300 kg/hac

2 5–7   2.5–3
Hydroseeding: seed, fertilizer, binder,
wood fibre mulch at 2000 kg/had

4 6–9   3–3.5
Hydroseeding: seed, fertilizer, binder,
mats held with staples or pegse

3 4–10   14–249

a Relative erosion control effectiveness rating after application before and after plant establishment;
where 1 = minimal, 10 = excellent. Mulches and binders provide temporary erosion control benefits
before plants are established.

b Cost relative to treatment by hand broadcasting.

c For helicopter seeding with 100–200 kg wood fibre mulch to aid application, relative cost is 3 to 3.5.

d Minimum mulch application rate for a true mulching effect to hold moisture and moderate temperature.

e Cost can vary considerably, depending on the specific mat product and site conditions; effectiveness
rating assumes good mat contact with the ground surface.



Timing

The best time to seed is immediately after soil disturbance, when the seedbed is in good condition. Climatic factors affecting seed survival and plant establish-ment should be considered when planning rehabilitation so that soil and climatic conditions are optimal for vegetation establishment.

Spring seeding

Successful spring seeding in coastal British Columbia allows a full growing season for plant establishment before extended winter rains. In the interior, early seeding can allow a full, or large part of a growing season, for establishment before the onset of winter.

Spring seeding should occur as soon as possible after access becomes available. In some areas there is the possibility that heavy rains or runoff will wash away the seed, but the greater danger is that there will be insufficient time for plants to establish before the onset of extended dry periods. Shallow mulch applications can provide some protection against desiccation. The typical window for spring seeding in coastal British Columbia is from mid-March to early June. In the interior the proper time to seed varies considerably.

Summer and fall seeding: interior British Columbia

Seeding at any time may fail if weather conditions turn unfavorable. However, late summer or early fall are preferred seeding times where fall rains occur early and there is sufficient time to establish a good grass cover. The risk of drought damage is greatly reduced in fall seeding. If fall rains happen to be late and temperatures drop before the grass germinates, there is still an opportunity to establish in the spring. Seeding should occur very near to the time that rains are anticipated, or even after they have begun, to reduce seed loss. In areas where rains do not start until late in the season, consider winter or early spring seeding.

Mulch application may provide some protection for germinating seeds in late season, with attendant higher cost.

Winter seeding: interior British Columbia

Winter seeding is widely practised in the interior and refers to the practice of seeding so late in the year that seed germination will not occur until the following spring. In its most extreme case, seed can be applied onto snow cover. Seed is very visible on snow, and excellent results can be achieved. The advantages of winter seeding are that the dormant seeds are protected by snow and will be in place to begin growing as soon as conditions are suitable in the spring. The disadvantage is that certain conditions can result in high seed loss, and that legumes seeded in early winter may suffer from poor inoculation with Rhizobium. Snow melt can sometimes concentrate seed into low areas.

Consult with an expert in your area; range agrologists usually have considerable experience with seeding in the interior. This technique can also be applied to sites of late snowpack areas that experience a short time period in spring between snow melt and summer drought.

Fall seeding: coastal British Columbia

Fall seeding is best for late snowpack areas with a short time period in spring between snow melt and summer drought. While fall seeding is vulnerable to winter erosion if not well established, plants have a head start on seeds sown the following spring and can take advantage of early spring growing conditions. If a full-scale seeding operation is not practical during the critical late-season period, at least a "temporary erosion control" seeding should be carried out with fall rye to provide the exposed soil surface with immediate protection against erosion.

In the fall, seeding should occur soon enough for germination and establishment – at least 30 days before the first heavy, erosive rain (50 mm or greater in 24 hours). Normally, seeding can occur from late August through mid-October in coastal British Columbia, depending on the season. When soil is extremely dry after an extended drought, it is best to seed after some rain has moistened the soil.

Surface preparation

Slopes must be mechanically stable for long-term success of seeding. It may, however, be necessary to seed unstable slopes as an interim measure.

A good seedbed has small cracks and discontinuities that trap seed and provide good contact between seed and soil. This improves germination because it helps prevent the seed from drying out. Large clods and very rough surfaces do not make good seedbeds because the clods dry out before the seeds germinate and seed tends to collect in the lowest points, resulting in very patchy distribution. Seedbed conditions are probably worst on compacted, smooth soil surfaces.

Seedbed conditions deteriorate with time after tillage because rainfall will cause crusting of the soil at the surface and filling the small cracks and pores that trap seed. For this reason, seeding should occur immediately after tillage.

A light surface treatment may enhance seedbed conditions where tillage has not been used for other reasons or where some time has elapsed since tillage. Many methods can be used, including chain drags or harrows pulled by ATVs or other light equipment. Hand raking may be as effective and efficient if work crews are available or if only small areas need to be treated. Consider the size of the seed as a guide to the degree of surface roughness required.

Conifers

Conifers may be planted as container stock or, where available, bare root stock, or direct seeded. Direct seeding of conifers has fallen out of favor as a general reforestation technique, partly because of problems with seed predation. Nevertheless, it may be used successfully in rehabilitation work and in some cases may help to establish trees where rooted seedlings are difficult to establish, for example, in some cases of fine-textured, compact or cold wet soils.

For conifer species selection, early successional species are generally preferred for disturbed mineral soil. Use of mulch mats around trees can help reduce competition from seeded ground cover where moisture stress exists. Conifers can be planted anywhere sufficient mineral soil is present, but will only grow to merchantable size where soil productivity has been successfully restored.

For coastal British Columbia

The following guidelines apply:

For interior British Columbia

The following guidelines apply:

Shrubs and hardwoods

For shrubs and hardwood trees, select species based on rehabilitation goals and site conditions. A mixture of plants will increase the chance of success. Tables A2.4 and A2.5 list some native species that have been successfully propagated and are suitable for erosion control in coastal and interior British Columbia. Operational experience with most of these species is limited, and availability may be a problem, although the situation is improving. First-year survival rates as low as 50% are considered normal for unrooted cuttings; survival of rooted cuttings and seedlings may be over 90%.

Table A2.4. Native shrubs and hardwood trees
for achieving site rehabilitation and erosion control
for coastal British Columbia sites

Latin name Common name

Alnus rubra Red alder
Alnus crispa ssp. sinuata Sitka alder
Betula papyrifera Paper birch
Cornus stolonifera Red-osier dogwood
Holodiscus discolor Ocean-spray
Philadelphus lewisii Mock orange
Populus spp. Poplar
Physocarpus capitatus Pacific ninebark
Rosa spp. Rose
Rubus parviflorus Thimbleberry
Rubus spectabilis Salmonberry
Salix spp. Willow
Sambucus spp. Elderberry
Spiraea douglasii Hardhack, pink spirea
Symphoricarpos albus Common snowberry




Table A2.5. Native shrubs and hardwood trees for achieving site
rehabilitation and erosion control for interior British Columbia sites

Latin name Common name

Alnus crispa ssp. sinuata* Sitka alder
Betula papyrifera* Paper birch
Cornus stolonifera* Red-osier dogwood
Philadelphus lewisii Mock orange
Populus balsamifera ssp. trichocarpa* Black cottonwood
Physocarpus capitatus, P. malvaceus Ninebark
Rosa acicularis, R. gymnocarpa, R. nootkana* Rose
Rubus parviflorus Thimbleberry
Salix spp.* Willow
Sambucus racemosa Elderberry
Spiraea douglasii Hardhack, pink spirea
Symphoricarpos albus* Common snowberry

*Species that are easy to collect and propagate, and that produce the best growth.


Consider the following general guidelines when selecting shrubs for rehabilitation:

Coastal sites

Sitka alder is recommended over other species. It may grow 3–4 m tall, with 80–95% survival after approximately 10 years when grown from either PSB 313 plugs or bare root. Survival of other species ranges from 50 to 80%, with growth of 50–200 cm after approximately 10 years. Where establishing conifers is a secondary objective, red alder can be used. It colonizes naturally in disturbed areas where there is a seed source, but this can take years. When an alder seed source is present, monitor natural establishment rates after seeding grasses and legumes because planting alder may not be necessary. Do not treat alder for conifer release on landslides, except on gentle slopes in the deposition zone.

Interior sites

For shrub species, Sitka alder (a nitrogen fixer) is adaptable to all interior sites, on both cut and fill slopes. All three species of rose are hardy and show high survival rates, but fastest growth occurs on fill slopes. Willows perform well on a variety of sites as small and large bare root stock and rooted cuttings. Common snowberry grows and spreads best on fill slopes. Mock orange and ninebark are confined to south and southeast facing sites in the interior. Red-osier dogwood is suitable on wet sites, seepage spots, and along the apron of wet cut slopes. Ribes species and elderberry are easy to establish on wet sites. So far, only limited success has been achieved with thimbleberry (naturals outperform planted specimens) and hardhack (pink spirea).

Hardwood trees may be planted as a variety of sizes of container stock, or where available, bare root stock, or direct seeded. Several seed collecting companies and nurseries in British Columbia supply a wide variety of materials for hardwood species. Depending on the site conditions, many species (including paper birch, trembling aspen and cottonwood) have the potential to grow, and some may produce commercially valuable trees in addition to having browse, wildlife habitat and other values. Black cottonwood can be propagated from stem and root cuttings; aspen should be obtained from a nursery because it is difficult to propagate. Black cottonwood performs well on road berms and fill slopes. Generally, aspen is found in drier ecosystems, birch on moist sites and cottonwood in wetter sites. Consult regional biogeoclimatic ecosystem classification field guides and ecologists in your area for appropriate species selection. High stocking rates may be used (e.g., 5000–10 000 stems per hectare) on severely disturbed sites, to promote organic inputs from roots and leaves into the soil.

Propagation

Several nurseries propagate shrubs and hardwood trees from seed or cuttings. You can also collect native poplar and willow and plant rooted cuttings. Figure A2.2 gives guidelines for collecting, handling and planting local materials.

Timing

Hardwood species should be planted during the dormant season (on the coast, plant in February and March if the snowpack and soil conditions permit, or October to November, before the ground freezes). Timing in the interior is largely dictated by elevation, snowpack, freezing or thawing of the soil, and access. Early planting is feasible at lower elevations. The alternative is fall planting until freeze-up. The propagation schedule of willow should be coordinated with the time of planting at a particular site: bare root stock left in the nursery too long may grow too large to handle. Both shoots and roots of bare root willow can be pruned to accommodate the large planting hole.

Figure A2.2. Collecting and planting Populus and Salix unrooted cuttings (from Beese et al., 1994, after Chatwin et al., 1994).

Density and pattern

Planting density for erosion control depends on the site characteristics and objectives. Grid plantings may vary from 3<–>3 m (1110 stems per hectare) to 1<–>1 m (10 000 stems per hectare) for critical sites. Grid plantings provide even coverage of the entire area. For problem sites with few suitable planting spots, planting in favorable microsites is more important than adhering to rigid spacing.

Test results of a 1<–>1 m grid planting after nine years showed that revegetating fill slopes with native deciduous species can produce a dense, vigorous cover. Cut slopes are more difficult to handle due to the compact, nutrient-poor bare mineral soil medium. This is especially true where surface erosion is in progress. In the absence of surface erosion control, the root collars of planted shrubs and trees may be exposed and the plants may fall over. Even if the surface soil is not lost and the plants survive, plant development will be retarded. In comparison, even a modest grass-legume cover on the slope surface will significantly enhance the development of planted shrubs and trees evidenced by field tests.

Row or hedge-like plantings can be effective for catching raveling debris. Rows may be 2–10 m apart, with plants spaced 0.25–0.5 m apart within rows. (Rows 3 m apart, with spacing of 30 cm within rows would require 11 000 plants or cuttings per hectare.)


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