Create a mini-forest

Forest creation best practice: translating a stormwater technique for the suburban lot

Meghan Fellows, CERP Project Manager II/Restoration Ecologist, Fairfax County Stormwater Planning Division  6/8/2021

Has the concept of a mini-forest ever caught your eye?  Are you ready to take your home landscape to the next level?  Might a mini-forest be a good option?  Japanese botanist Akira Miyawaki has turned wastelands into incredibly dense native forests, even in suburban yards.  The concept of “nucleation” made it into scientific literature in 2012 (Collins and Hall, 2012).  Mt. Cuba Center, a native plant garden in Delaware, has been planting their version of mini-forests since 2015 (Mt. Cuba starts 20-year study of 6 reforestation styles.)  And Fairfax County’s Stormwater Planning Division planted its first “Superclumps” in 2017.  Although they all have different names, and variations on the details of the technique – what these practices have in common is a willingness to try a different approach to reforestation, plant dense hot spots of super high diversity, and see what happens.

Planting densely goes against most of our cultural knowledge of how to make a forest grow.  This method does not ignore the basic tenets: (1) plants need space, (2) plants grow, so you must plan for their mature size, and (3) plants need resources. However, if we work with some of what is observed in a natural forest, such as: (1) plants root at different depths and in different patterns, (2) plants can grow up as well as out, and (3) a natural forest has plants that are very tightly spaced -- then we can envision how a mini-forest might look in NOVA:

  1. Multi-level with herbaceous, shrub, understory and overstory

  2. Promotes natural regeneration and shade

  3. Maximizes biodiversity

Figure 1 In the middle of the third growing season, the mini-forest planting technique created dense shade in a short amount of time. July, 2020.  Fairfax County photo.

In the middle of the third growing season, the mini-forest planting technique created dense shade in a short amount of time. July, 2020.  Fairfax County photo.

Preparation and Maintenance

Although native plants are relatively low maintenance, a native planting that is working well will attract birds, and birds disperse invasive species.  To minimize invasive species spread, plan on preparing your site properly.  Begin with the removal of turf grass by scalping, solarization, or at least two applications of herbicide six weeks prior to planting.  Spread mulch at planting and throughout establishment, usually one to three years.  Prune the woody material after two years of growth and remove any vines for the first three years.  Finally, complete any follow-up care needed to maintain the forest free of non-native invasive plants.

Landscaping, restoration planting, and other land management techniques do not end once plant material is installed. Successful planting efforts require a maintenance plan, sometimes both short-term, during the establishment period, and long-term.  To minimize long-term maintenance and the loss of valuable plant material, choosing the right species for the landscape is exceptionally critical.  Fortunately, with this planting technique, you can push the boundaries a bit knowing that the context for the planting will change quickly (a full sun location can be full shade in less than three years).

Forest structure

A natural, healthy forest has multiple layers of plants.  The layers create structure which contributes to forest function. Additionally, the type of plant species in each layer matters, e.g., if you want spicebush swallowtails, you’ll need to plant two spicebush. Typically, a healthy forest is comprised of five layers: overstory, understory, shrub, herbaceous layer, and the forest floor/soil. Establishing a forest from a disturbed landscape means management of each layer to put each on the trajectory to recovery.

Overstory Trees

Overstory trees are the backbone of most of our Eastern Temperate Deciduous forests.  In Fairfax, these are predominantly the Oaks and Hickories.  While it is relatively easy to source oaks from local nurseries, hickories have a long tap root which make them a logistical challenge to produce in a nursery and a poor transplant option.  

 

A typical home landscape might even exclude overstory trees like oaks and hickories, as they can easily grow to be over 80 feet tall.  However, it is common to see residential properties with trees this tall, as those trees were present when the house was constructed. Some landscapers avoid canopy trees next to the house, but others simply recommend an inspection by a certified arborist every two years. In any event, tall trees should be planted at least 10 or 20 feet from the house." Sighting a mini-forest in your home landscape should be far enough away from structures that they don’t become a future hazard.

Fast-growing overstory trees

One would not normally invest money into early successional trees as they grow fast and die young (20-40 years), but these ‘old-field weeds’ are essential for the success of mini-forest plantings. These species create quick shade and provide relatively instant gratification with their immediate presence.  It is possible in the long-term that these species can become too aggressive, except, in general, these species need light and disturbance to germinate and grow. Light and disturbance should not be present in a mature mini-forest.  After three years, some of these plants can reach up to 30-40 feet tall, although mature height can be over 80 feet.

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Fast-growing trees like black locust, sycamore and tulip tree, create rapid visual impact, heights of 20-30 feet are not uncommon after 2-3 years. Three-year old, 15' diameter mini-forest planting, July 2020.  Fairfax County photo.

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Understory trees

These trees are smaller, showier, and tolerant of partial sun, but also adapted to full sun conditions. Typically, at 15-30 feet high at maturity, these species provide the valuable mid-canopy structure that boosts forest biodiversity. Many of these species are already used as native trees for the home landscape.

Shrubs

At 2-15 feet high, shrubs are at the prime height to be heavily browsed by deer and are therefore frequently missing from our local forests.  If you find a forest in Fairfax with a shrub layer, it is usually dominated by non-native shrubs, which are less palatable to deer.  The pockets of native shrubs that are found, however, are exciting and important contributions to the overall forest health.  

A mini-forest is an ideal setting for these shrubs.  The shrub component also increases stems and creates a visual barrier to curious/hungry deer before they enter the mini- forest.

Understory herbaceous

A forest floor is not a solid carpet of understory plantings.  Usually, the forest floor consists of a thick layer of leaf litter (2-4 inches), leaf mold, fine woody debris and duff sitting on top of a rich organic layer of soil.  A healthy forest floor is inhabited by relatively sparse plants. The herbaceous layer dapples the forest in patches, often surrounds a tree fall, and is not expected to grow fast or spread.

Buffer / perimeter meadow

A forest must have an edge, and the edge can provide a transition into the adjacent land use.  With a sharp edge, the forest will create a transition zone of its own, reducing the footprint of the forest.  Treating the buffer zone is an important part of the management of the mini-forest.  In the home landscape, the transition from forest will be to a sidewalk, lawn or built structure, so treating the edge as a meadow will protect the forest, the built environment and allows for another planting zone.  If more trees and a greater footprint of the forest are desired long-term, use fewer grasses within the buffer area and less frequent or no-mow styles of management.

The mini-forest

A planting should have multiple species from each of the categories mentioned above.  Some Miyawaki forests add three plants for every three feet.  Fairfax County plantings are slightly less dense with one plant for every three feet, or an average plant spacing between 2.2-3.3 feet. At its simplest, a 15’ diameter circular planting could have 3-4 overstory trees, 3-5 fast growing trees, 3-4 understory trees, 8-10 shrubs, sufficient mulch or understory plantings to extend two feet beyond the perimeter of the nucleus, sufficient meadow or planned management to extend eight feet beyond the forest edge. Use a simple multiplication factor of 2 or 3 to enlarge the planting area. Any gaps between the circles should have additional shrubs or understory herbaceous to fill in the voids .

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Example mini-forest based on the 15-foot diameter circle x 2. Graphics by D. Sette, Fairfax County.

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Establishing a meadow buffer outside of the nucleus prevents weeds and allows for additional biodiverse habitat. One year old planting July 2020.  Fairfax County photo.

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A triangular shaped mini-forest possibility, note no understory in this variation. Graphics by A. Darr, Fairfax County.

An alternate planting layout based on an equilateral triangle, modifies the number of plants, depending on the length of the side.  Fast growing trees and shrubs anchor the points and sides of the triangle with overstory and understory trees creating alternating triangles within.  Multiple triangles can be abutted, negating the need for bonus shrubs or herbaceous to fill in any gaps.

Plant size

Installed plant size has a huge impact on initial cost. Generally, it’s been observed that smaller trees have higher success rates in the long-term as they adapt more quickly to site conditions.  They are also easier to install, weigh less, require smaller holes to be dug, and they are generally less expensive.  Understory trees and Fast-growing overstory trees should be planted at ¾” caliper.  These larger plants will be a physical presence that will be both visually appealing and provide a physical barrier to deter deer browse.  Overstory trees should be planted no larger than 3/4” caliper.  In the case of hickories, it is common to find bare root plants due to their long tap root and constraints with growing in containers.  Shrubs and herbaceous material should be appropriately sized for the container that they are purchased in.  A full, 30” tall shrub in a small pot is likely to have root issues, whereas a 6” shrub in a large pot may be a healthier plant due to the generous room within the container for the root ball to develop.  The healthier the shrub and herbaceous material is when installed, the higher the chance of a successful planting.

Fencing

Welded wire fencing at least four feet, but preferably five feet, tall installed with oak stakes, will help deter deer, beaver and human interference during establishment.  Fencing should be placed two feet from the outer edge of the plant material to allow room for growth without injury.  Maintain the fencing to free of invasive and native vines during establishment.  Remove the fencing when the plants are large enough to tolerate deer browse.  Given the relatively small size of the mini-forests, one row of fencing should be sufficient to protect the plantings, but if the diameter is greater than 30 feet in the smallest dimension, consider using two rows of welded wire fence, three feet apart and/or 8-10-foot-tall deer fencing.

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Single 15' diameter with Juniperus virginiana. Human and beaver threats have set this planting back a little. Three years' old, July 2020.

The plant species

Single family homes are the largest component of land use in Fairfax County.  The idea of restoring a forest with pristine vegetation is both romantic and slightly impractical (a home lot must serve the function of the family/owner, support infiltration and limit rain runoff from the impervious surface of the house and accessory structures, as well as provide other needs as individual as the lot itself).  Recent studies by Doug Tallamy and his students have suggested a good target for suburban plots of land should be 70 percent cover of native plants, as this can be sufficient to support generalist native bird populations.  As most lawns are non-native turf grasses, the non-lawn portions should be primarily native species to maximize ecosystem benefits.  During the plant selection process, it is important to analyze site conditions, use needs, and the native vegetation community, as well as the homeowner’s additional priorities and needs.  Below are four suggested planting palettes.  These can be mixed and matched.

Species to avoid

Some plants are known to be “runners,” meaning that they will spread outside of the dedicated forest plot (see the first image above).  Other species can grow very tall and may drop fruit (e.g., sweet gum) which can potentially increase seasonal maintenance.  The avoidance of runners, tall species, or fruiting plants are personal preferences which can be considered during species selection.

Plant palettes

Ecological superheros

This palette should be no closer than 20 feet from the house, as native forest trees can be quite tall at final height.

Pollinator buffet

These species are more light loving, and to maintain light, you may will want to avoid or limit the overstory tree component.

Forager Fantasy

Wild foods are for wild animals. Create this forest type to ensure a sustainable harvest that you too can enjoy. Note species with a * aren’t known to be edible to humans.

Riparian water hog

Forest establishment near a stream has some unique opportunities.

Special thanks to Suzy Foster, Danielle Sette and Alex Darr, Fairfax County employees.

See these mini-forests in action.

  • Churchill Road Park (multiple-aged stand of mini-forests, with single, multiple and A-type patterns)

  • Lewinsville District Park 

  • Rock Hill District Park

  • Huntsman Lake (beaver!!)

  • DPWES-stormwater@fairfaxcounty.gov

Multiple mini-forests at this site should eventually grow together, 2.5 years after planting, June 2020.  Fairfax County photo.

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