Sunday, June 16, 2019

Forest Gardening

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Travis Beck, Department of Horticulture and Crop Science, The Ohio State University

Martin F. Quigley, Department of Horticulture and Crop Science, The Ohio State University

Forest gardening offers a new approach to home landscaping in both urban and rural areas of Ohio. Forest gardens are attractive, low-maintenance gardens that combine food production with ecological function.

What is a forest garden?

Quite simply, a forest garden is a garden modeled after

a natural forest. Forest gardens usually have a diverse collection of plants arranged in multiple layers above a deep mulch. These plants produce flowers, food, and other products for human use. The plants function together, along with the insects and other animals that come to inhabit them, as an ecological system (Figure 1). Forest gardening is a term coined by Robert Hart of Shropshire, England. Indigenous peoples throughout the tropics, however, have created similar gardens for centuries. These gardens, known as tropical home gardens, include a diversity of crops arranged in multiple layers, and in many ways mimic the structure and ecological function of the surrounding natural forests. Robert Hart and others have applied the same principles to the design of gardens modeled after temperate deciduous forests. Forest gardens are especially appropriate for Ohio as up to 95% of the state was forested before its settlement, and because there are many native plant species that can be used successfully.

How to design a forest garden

In a natural forest, plants belong to one or more layers. There are the high canopy trees, and an understory of shorter trees. Beneath the trees is a layer of saplings and shrubs and beneath that a layer of herbaceous plants. Just above the forest floor are groundcovers. Finally there are the vines, which grow up through all the layers. The key to designing a forest garden is to think vertically—how many plants can I stack into this space? Suggestions for plants for each of the layers can be found in the plant list below. For a new garden area or yard, plans can be drawn up from scratch. Forest gardens can also be built around existing trees or shrubs, or in established woods. For very small spaces, such as an urban back yard, the canopy layer can be omitted, and the design built around a dwarf fruit tree.

The plant layers in a natural forest are not uniform across space. Different species live on high slopes as opposed to in a river bottom, for example. Forest composition is different at the edge of a clearing from the center of a large wooded patch. Similarly, in a forest garden we design with the environment and according to our specific uses. If one corner of a forest garden is more poorly drained than another, that corner should include plants that can tolerate having “wet feet.” A forest garden can be set up like a forest edge, with low growing species to the sunny side and taller plants in the rear. Forest gardens can perform structural functions, such as screening an unpleasant view or cold wind, or, if mostly deciduous, can shade a house in the summer and let the sun in to warm it in the winter. Aesthetic concerns are also important. The principles of grouping plants, achieving balance, and creating harmonious designs that are covered in basic landscape design texts still apply to the forest garden.

Natural forests grow magnificently with no human intervention. Forest gardens too seek to provide many, if not most, of their own needs. A sheet mulch (explained in the installation section) will keep the plants moist without the need for much additional irrigation, and, as it breaks down, will furnish the plants with valuable nutrients needed for their growth. To increase the fertility of the system, forest gardens often include nitrogen-fixing plants and plants grown specifically to provide mulch. Forest gardens also often include insectary plants to attract and support beneficial insects that can reduce pest populations. Perennial and self-seeding plants are emphasized in forest gardens because they reduce the need for the annual chores of starting and planting out seedlings. Designs may also include habitat for birds (who spread seeds, provide fertilizer, and eat insects), toads (who eat insects), and snakes (who eat rodent pests).

Like natural forests, forest gardens are, to an extent, self-designing. The gardener may introduce more seeds and plants than will eventually survive. Only those that are well-suited to the environmental conditions present in that place at that moment will thrive. Also, a forest garden is designed to evolve over time. The plants that predominate at the beginning will likely fade away as others grow to take their places.

The plant list presented here is not meant to be exhaustive. Any plant that can grow in the cultural conditions of a forest garden can be included. This includes strictly ornamental plants and certain vegetables. Many cool season vegetables will appreciate the shade and moisture of a forest garden. Heat-loving summer vegetables can grow in a forest garden in its early years. Or they can be grown in sheet mulch on the garden’s edge.

Plants for an Ohio Forest Garden
Common NameScientific NameUses
Canopy
BeechFagus grandifoliaNuts
ButternutJuglans cinereaNuts
Shagbark HickoryCarya ovataNuts
Sugar MapleAcer saccharumSyrup
White OakQuercus albaNuts
Understory
American PersimmonDiospyros virginianaFruit
AppleMalus pumilaFruit, flowers
CherryPrunus spp.Fruit, flowers
Cornelian CherryCornus masFruit
CrabappleMalus spp.Fruit, flowers
HazelnutCorylus spp.Nuts
Kentucky Coffee TreeGymnocladus dioicaN-fixer
Paw PawAsimina trilobaFruit, flowers
PearPyrus communisFruit, flowers
PlumPrunus domesticaFruit, flowers
ServiceberryAmelanchier spp.Fruit, flowers
Witch HazelHamamelis virginianaMedicinal, flowers
Shrub Layer
BlackberryRubus occidentalisFruit, flowers
CurrantRibes sativumFruit
ElderberrySambucas nigraFruit, flowers
False indigoBaptisia australisN-fixer
GooseberryRibes uva-crispaFruit
RaspberryRubus idaeusFruit, flowers
RoseRosa spp.Medicinal, flowers
Siberian Pea ShrubCaragana arborescensN-fixer, flowers
Herbaceous layer
ArugulaEruca vesicariaEdible
ChamomileChamaemelum nobileTea, flowers
ChivesAllium schoenoprasumEdible
ComfreySymphytum uplandicumMedicinal, mulch
CornsaladValerianella locustaEdible
DillAnethum graveolensEdible, insectary
FennelFoeniculum vulgareEdible, insectary
GarlicAllium sativumEdible
KaleBrassica oleraceaEdible
Lemon balmMelissa officinalisTea
LettuceLatuca sativaEdible
LovageLevisticum officinaleEdible
MintMentha spp.Edible
New Zealand SpinachTetragonia expansaEdible
OnionAllium cepaEdible
ParsleyPetroselinum crispumEdible
RhubarbRheum rhabarbarumEdible
Salad burnetSanguisorba minorEdible
SorrelRumex scutatusEdible
SpinachSpinacea oleraceaEdible
Stinging NettleUrtica dioicaEdible, mulch
Flowering Ground Covers
StrawberryFragaria spp.Fruit, flowers
NasturtiumTropaeolum minusEdible flowers
VioletViola spp.Edible flowers
Vines
GrapeVitis viniferaFruit
Hardy KiwiActinidia argutaFruit, flowers
HopsHumulus lupulusMedicinal
Scarlet Runner BeanPhaseolus coccineusEdible, N-fixer, flowers
WisteriaWisteria floribundaN-fixer, flowers

How to install a forest garden

The ideal time to start a forest garden is in the fall. Trees and shrubs can be planted if they are already on hand. Plant trees and shrubs even with, or slightly above the existing ground level. Next, lay down a sheet mulch (Figure 2). If trees and shrubs are not ready, begin with the sheet mulch.

The first step in preparing a sheet mulch is to knock down all existing vegetation. Knocking down is different from pulling up or tilling under. Depending on the vegetation, use a mower, weedwhacker, bush hog, scythe, pruning shears, or your feet to break off all undesirable vegetation at or just above ground level. Sod can be left in place and the sheet mulch laid on top of it. Substantial woody vegetation can be removed for chipping and later use as a path or part of the mulch. Thick woody roots may also be removed. These can be brought back later to serve as wildlife habitat or mushroom logs above ground. Otherwise, leave everything to decompose in place.

If the soil is compacted, break it up with a spading fork (one with a metal handle works best), a U-bar, or, for large areas, a subsoiler. With the fork, insert the tines to their full depth and lean on the handle. The object is to break up the soil without turning it over.

Now is the time to introduce soil amendments. A soil test will show whether it is necessary to adjust the soil pH with lime (to increase alkalinity) or with sulphur (to increase acidity). Also add any nutrients in which the soil was shown to be deficient. An inch or two of compost will immediately add organic matter and nutrients to the soil and is the best all-around amendment. Next, add something rich in nitrogen to draw worms to the surface and aid in the decomposition of the carbon-rich layers that are going on top. Composted manure is an excellent source (add 1/2 to 1 inch), as are fresh vegetable scraps (a local grocery store or restaurant may be able to provide these in quantity). All of these amendments are laid directly on top of the soil one after the other and not worked in.

Next comes the light barrier (Figure 3). The object of

Figure 3
Photo by Travis Beck.

this layer is to prevent the germination of weed seeds in the soil and to provide a physical barrier to any weeds that do get started. Two readily available materials for this layer are newspaper and sheets of cardboard. Remove any glossy color pages from the newspaper, as these contain metallic inks. Remove tape and staples from the cardboard as much as possible. Any solid 100% organic material can also be used, including old cotton clothes or flakes of hay. Lay down newspaper and cardboard so that the edges overlap substantially. The newspaper should be anywhere from 4 sheets to 1/2 inch thick depending on the anticipated tenacity of the weeds underneath. Around the trunks of trees and the bases of shrubs, tear a section of newspaper or cardboard halfway through, and slide the two edges of the tear around the trunk. Tear a second sheet in the same way and place it perpendicular to the first. It may be necessary to wet newspaper occasionally with a sprinkler or hose to keep it from blowing around during the installation process. The light barrier is the key to the success of the sheet mulch. It takes some time to lay it with care, but it can save many hours of weeding later. Do not be alarmed if the newsprint on the light barrier is still legible as much as a full season after installation. Eventually it will break down and contribute to the organic matter in your soil.

On top of the light barrier, place another layer of nitrogen-rich manure or vegetable scraps. Then lay up to 12 inches of bulk mulch. Bulk mulch can consist of leaves, grass clippings, straw, wood chips, pine needles, branches, reeds, or any uncomposted rough organic matter. The mulch can be placed in discrete layers, or mixed together. It need not be uniform. Leave some space around tree trunks and the bases of woody shrubs. Otherwise rodents may come to live in the mulch and nibble on the tree trunks. If rodents are a likely problem, cut a slit in a metal can and place it gently, like a sleeve, around the base of the tree. Two considerations are important for the bulk mulch. One, avoid material that may contain seeds, such as hay and weeds. These materials can be included in a sheet mulch, but they should go beneath the light barrier. Two, creating a balance of carbon-rich “browns” and nitrogen-rich “greens” will help the mulch break down more rapidly. The ideal ratio of carbon to nitrogen is around 30:1. Composting books have much information on how to achieve this ratio with different materials. An easy recipe is to mix fallen leaves with fresh grass clippings and apply this as the mulch. If the crucial C:N balance has not been achieved, the mulch will still break down, but it will just take longer. To hasten the process, use compost or manure teas as described below.

On top of the bulk mulch one can place a final layer of hard mulch to make everything look tidy. This hard mulch could be straw, bark mulch, wood chips, or pine needles.

It is best to let a sheet mulch compost for a period of time after installation. If the mulch has been installed in the fall, it will partially break down over the winter to create rich garden soil. If it has been installed in the spring, it may only be possible to wait a few days or weeks before planting.

Planting the forest garden

Ideally, trees and larger shrubs have been planted before the application of the sheet mulch. If not, remove a section of mulch, dig a hole (place the dirt on a tarp or piece of plywood), plant the tree, spread any excess dirt in the exposed area, and replace the mulch as neatly as possible. The same procedure applies to any large herbaceous plants being installed. For transplants, planting is done with a trowel, a knife, and a bucket of topsoil or compost. If necessary, scrape away a small area of the surface mulch. Use the knife to cut a slit or an X in the light barrier. Insert the trowel into the slit and dig a small pocket for the roots of the transplant. Place the roots of the transplant into the pocket. Fill around them with topsoil or compost. Return the mulch so the transplant is just sticking through. Large seeds, such as beans and squash, can be placed directly in the mulch if it is well broken-down, or in slits in the light barrier. Plant far more seeds than you want plants, since many may rot in the mulch or be eaten by birds. Fine seeds, such as those of carrots and lettuce can be germinated in patches of top soil or compost which are placed in the mulch. Clear the mulch aside from an area and cut holes or slits in the light barrier. Cover with top soil or compost, up to the level of the mulch. Sow the seeds and care for as in a garden bed. In future seasons, plants such as dill and arugula may reseed themselves. This process can be encouraged by placing a layer of compost around the plants as they begin to drop their seeds.

Maintaining a Forest Garden

The needs of a forest garden are the same as those of any garden. If well designed and properly established, however, a forest garden takes good care of itself.

Watering

Trees and shrubs in their first season may need occasional deep watering. All other plants should be watered when the soil beneath the light barrier dries out, or when the plants show signs of water stress. As the deep mulch protects the soil from wind and sun, forest gardens typically require only infrequent watering. Watering can be done with a gentle flow straight from the hose, with overhead or handheld sprinklers, with drip irrigation or soaker hose, with a watering wand, or with a watering can.

Weeding

If the sheet mulch is done properly, very few weeds will appear in a forest garden. Those that do appear can be pulled and left on top of the mulch to dry, or tucked underneath the mulch to rot.

Fertilizing

Most of the nutrients needed by the plants in a forest garden will be provided by the mulch as it breaks down. If a soil test reveals the need for specific nutrients, provide those at the time of the initial sheet mulching. Especially in its first season, a forest garden mulch may “lock up” nitrogen from the soil in its own process of decomposition. The most important form of fertilization, then, is to provide nitrogen to allow for the decomposition of the mulch and the release of nutrients to the plants. The best way to provide this nitrogen is in liquid form. This can be accomplished by preparing a manure or compost tea. Many recipes for such teas exist, requiring various levels of equipment and sophistication. A simple recipe is to fill a bucket or trash can 2/3 full with manure or compost. Fill the container with water to a few inches below the rim. Cover with a sheet of plastic tied around the rim and leave in the sun. Stir every day or two until the contents are rank and bubbly (around a week). Strain out the liquid, dilute at a concentration of 1:10, and water the entire garden with this “tea.” Plants in a forest garden can also be fertilized with a foliar (leaf application) fertilizer.

Mulching

Mulching is the most important maintenance task in a forest garden, and, as seen above, reduces the need for the watering, weeding, and fertilizing that a traditional garden requires. Any bare spots that appear in the mulch should be covered up with fresh material. If a large number of weeds breaks through in one area, remove the bulk mulch, add an additional layer of light barrier over the entire area, and replace the bulk mulch. If you are growing nitrogen-fixing or mulch crops (e.g., Siberian pea shrub, comfrey), these can be cut back occasionally and either incorporated into a compost tea, or spread on the mulch surface to add nutrients. At the end of the season, spread several inches of leaves or other mulch material over the entire garden (be sure not to bury herbaceous perennials!), and leave it to break down over the winter.

Clean up

As a forest garden includes many perennial plants, clean up at the end of the season can be less of a task than in annual flower and vegetable gardens. Rather than pulling plants up, cut them off at ground level, which disturbs the soil less. The tops of the plants can then be incorporated into the mulch, or set aside for compost.

Conclusion

Forest gardens require an intensive effort up-front. After this, they have the potential to care for themselves for years as they evolve into a self-sustaining ecosystem. Forest gardens can provide food, flowers, wildlife habitat, wind protection, cooling, and aesthetic enjoyment in almost any home landscape in Ohio.

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