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SULIS - Sustainable Urban Landscape Information Series.
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Planting Under Existing Trees

Description/Purpose:

People are interested in under-planting existing trees for a variety of reasons. Perhaps they have struggled to keep turf grass alive under a tree canopy and are looking for a better solution. Perhaps they have a small yard and the area beneath the tree's canopy is critical to their being able to have flowerbed space in their yard. Perhaps they are tired of mowing or have admired blooming boulevards planted by others in their neighborhood and would like to follow suit. Whatever the circumstances, under planting trees with plants other than turf is an option that can be aesthetically pleasing and also beneficial to the tree, if done with certain practices and principles in mind.

Tools, Supplies and Equipment:

  • A garden fork
  • Small garden trowel
  • Garden gloves
  • Rope, a hose and / or spray paint
  • Organic mulch (shredded leaves, cocoa bean hulls, shredded bark or wood chips)
  • Slow release Nitrogen fertilizer
  • Roundup
  • A water source

    *Do not use large or power equipment, even if available!

Site Situations and Considerations:

Avoid Tree Root Damage

In all instances, care needs to be taken to minimize disturbance and damage to tree roots during the preparation and planting processes. There is a common misconception that most tree roots are deep and create a mirror image of the tree's crown. In reality, most roots are fairly close to the surface and reach even beyond the drip-line of the crown (Figures 1a and 1b). If the tree has turf growing (or struggling to grow) up to its trunk, this grass needs to be carefully removed before planting takes place. Studies have shown that young trees grown without turf beneath their canopies enjoy a more vigorous root system and grow larger and faster above ground, as well. Generous organic mulch rings under trees encourages this healthy growth. Under planting with appropriate herbaceous materials that don't compete aggressively with the tree's surface roots is also beneficial. Using either non-turf plants or mulch under trees helps keep soil from drying out quickly and eliminates thatch buildup and reduces soil compaction.

Figure 1a
Figure 1a. Most people's imagined configuration of tree roots - sort of a mirror image of the above trunk and branch system - which is quite deep and stops just outside the canopy drip line.

Figure 1b
Figure 1b. A more realistic view of a tree's roots. Most roots are relatively shallow (three feet or less deep) with most roots within 12 inches of the surface. Roots extend well past the canopy drip line.

Canopy Density

A tree's canopy density needs to be considered when choosing plant materials that will grow beneath it. The extremely dense canopies of certain trees, such as Norway Maples or Little-leaf Lindens, will not only block sunlight, but their umbrella like nature will also deflect rain. Only plants that are very shade tolerant can be used in these situations. Also, owners need to be prepared to supply supplemental water to such plants, not only at planting time, but until they become established and into the future during any time of inadequate water. Selective pruning to reduce canopy density may also be considered. Other good options for planting under trees (especially those with dense canopies) are spring flowering bulbs and / or spring ephemeral plants. Both of these groups of plants grow and flower before trees fully leaf out in the spring. They are, therefore, able to get adequate sun and rain during their early, showy season. They then die back and do not reappear until the following spring. Because they aren't visible year round, they are best over-layered with 6 inches of mulch after fall planting. They could also be interspersed with shade tolerant ground covers or other perennials.

Perennials versus Annuals

Because, ideally, tree root disturbance would be kept at a minimum, it is wiser to under-plant trees with perennial plants, rather than annuals, which require replanting every year. This way the disturbance happens only once and thereafter, the tree's roots and the roots of the perennials co-mingle and come to equilibrium.

Small versus Big

In an effort to disturb tree roots as little as possible, it is wise to plant small-size container plants, even if they will grow to a much bigger mature size. Using plants in 2-1/2 to 4-1/2 inch pots means needing to dig much smaller planting holes. Remember to space plants allowing for their mature size.

Plant Placement

Keep in mind the mature heights of plants as well as their mature width. The height of the tree and the height of its lowest branches will influence how tall the under-planting should be. If under-plantings crowd the lower branches, they will appear awkward and misplaced. A ranking of heights is also appropriate. If using plants of differing heights in tree under-plantings, the tallest plants should be used nearest to the trunk, stair stepping down to the shortest plants in the front (Figure 2a and 2b).

Figure 2a
Figure 2a. A poorly planned under-planting with plants that are too tall for the site. Also, small plants hidden behind taller plants.

Figure 2b
Figure 2b. A well-designed ranking of plants from tallest to shortest. This enables all plants to get light and to be seen.

For more information on design see Principles of Design in the Design section of SULIS under The Completed Landscape Design. For more information on plant selection based on plant height and width see primary elements of design and secondary elements of design in Elements of Design in the Design section of SULIS under The Completed Landscape Design.

Keep Existing Grade

Resist any urge to substantially raise the grade around a tree to facilitate under-planting. The impulse to add dirt around trees is strong, especially on trees with large shallow roots, such as Silver Maples. The addition of more than a few inches of soil is unwise. The added soil can reduce water and oxygen supplies to existing roots, which will lead to the slow decline of the tree. Neither soil nor mulch should be piled up on tree trunks. This can lead to adventitious roots that can become stem-girdling roots. It can also lead to rot at the base of the trunk. Never pile soil directly over existing turf and then plant. The buried turf and thatch will die and decompose, but in an anaerobic situation, it creates a "black layer" that is hard for roots to penetrate. See also, Protecting Existing Trees During Construction in the Implementation section of SULIS.

Root Toxins

Black Walnut trees (and the related Butternut tree) can be particularly problematic in coexisting with other plants. The roots of these trees produce a toxin known as Juglone, which can stunt, deform or even kill other plants. Plants in the nightshade family are most susceptible. These include the food crops of tomatoes, potatoes, peppers and eggplants. Ornamentals susceptible would be Nicotiana (flowering tobacco), Browallia (sapphire flower), Nirenbergia (cup flower) and Petunias. Since these plants are annuals and need full sun to do well, they are unlikely to be chosen as under-planting material, but anywhere the tree roots may reach, even beyond the drip-line, could be problematic.

Boulevard Trees

When planting under boulevard trees, be sure to allow room for yourself and your guests to easily get out of their cars and make their way to your home without treading on your flowerbeds. Grass or mulch pathways or flagstone or brick paths through the boulevard can make this easier. Remember too, that boulevard areas are usually a harsher environment for plants. Soils are typically poorer, hotter and drier than other areas and can be subject to road salt stresses. Therefore, some of the shade-loving, woodland plants that will do well in moister back yards will struggle on boulevards. Select tougher plants that can handle this tougher environment.

Step-by-step Process:

  1. Like most gardening projects, proper preparation is key to success and can also be the most time-consuming part of the experience.

  2. First determine the size and shape of the area to be under-planted. Mark the perimeter using a flexible material such as rope or hose. You can mark this edge with a can of spray paint for a more durable guideline.

  3. If turf is sparse or in little tuffs, you can remove it manually. Using a garden fork, being careful not to go any deeper than necessary, loosen any existing turf, including its roots. Tap these sod chunks briskly against something - the ground, a rock, a garden fork - so as to leave behind most of the soil in the ground where it came from. This is most easily done when the soil is neither too wet nor bone dry. Compost or dispose of the shaken turf chunks.

  4. If the project is done over a series of days, water well or temporarily mulch any area that is newly bare earth, to keep any exposed tree roots from drying out.

  5. If there are considerable areas of turf to remove, apply a non-selective, systemic killing agent, i.e., Glyphosate (brand name Roundup) following package directions. Since Glyphosate, the active lethal ingredient, is trans-located from the leaves of the plant to its roots, it will kill the surface plants, and yet, not harm or disturb the tree roots. Once the turf is dead, after approximately three days, plants can be planted directly into the dead turf area. The dead turf can then be covered with an organic mulch, 4-6 inches deep. The dead turn will eventually decompose and add to the soil's nutrients.

  6. During this bed preparation process, and while planting, keep the containers of plants waiting to be planted clustered together, out of direct sunlight. This helps keep the plants from drying out or becoming heat stressed.

  7. Using a small hand trowel, dig holes big enough to accommodate the roots of your under-planting materials. If you encounter a large root (2" diameter or more), move your planting hole to avoid the root. Cutting through smaller roots does the tree no real harm. New roots will grow from the point at which the old root was severed. Put a sprinkling of slow-release fertilizer in the hole, following package instructions. Loosen the plants from their pots, scuffing up their root ball a bit. Tuck them into the holes, firming soil around the roots so as to avoid air pockets. It's best to work in sections starting near the tree trunk and working away to the planting bed perimeter, lightly watering sections as you go. This will minimize compaction of the soil and keep things moist without being muddy.

  8. If plants are spaced far apart due to expected growth or spread, put down a layer of organic mulch over any bare ground areas from 4 to 6 inches in depth. Remember to keep mulch away from the tree's trunk. This mulch layer will keep down weed germination, retain soil moisture and reduce soil compaction.

  9. When all plants are planted, give the entire project area another good watering and good weekly watering until the plants are established. This is unnecessary if rainfall is adequate.


Plant Lists (not exhaustive)

Since each under-planting situation is unique, some trial and error may be necessary in choosing plants. Additional plant selection information can be found in the Plant Selection Program in the Plant Selection section of SULIS.

Larger, Woodland and Cultivated Perennials

Scientic name Common Name
Adiantum pedatum maiden hair fern
Arisaema triphyllum Jack-in-the-pulpit
Astilbe spp. astilbe
Athyrium niponicum 'Pictum'      Japanese painted fern
Brunnera macrophylla Siberian bugloss
Dicentra spp. bleeding heart
Dryopteris spp. shield fern
Epimedium xrubrum red barrenwort
Hosta spp. plantain lily
Liriope spicata creeping lily-turf
Polygonatum spp. Solomon's seal
Primula hybrids primroses
Pulmonaria spp. Bethlehem sage
Uvularia grandiflora merrybells

Low Spreading Groundcovers

Scientic name Common Name
*Aegopodium podagraria snow on the mountain
Ajuga reptans bugleweed
Asarum canadense wild ginger
Convallaria majalis lily of the valley
Galium odoratum sweet woodruff
Lamium spp. dead nettle
Pachysandra terminalis Japanese spurge
Tiarella cordifolia foam flower
Vinca minor periwinkle
Viola odorata garden violet
Waldsteinia fragarioides       barren strawberry
*Very aggressive

Spring Blooming Bulbs

Scientic name Common Name
Anemone blanda Grecian windflower
Crocus vernus Dutch crocus
Galanthus nivalis snowdrops
Muscari armeniacum grape hyacinth
Narcissus spp. daffodils
Puschkinia scilloides       striped squill
Scilla siberica Siberian squill

Plants More Tolerant of Boulevard Conditions

Scientic name Common Name
Sunny sites:
Achillea spp. yarrow
Asclepias tuberose butterfly weed
Aster spp. aster
Rudbeckia hirta black-eyed Susan
Salvia spp. salvia, sage
Sedum spp. stonecrop
 
Shady sites:
Alchemilla mollis         lady's mantle
Campanula spp. bellflower
Hemerocallis spp. daylily
Heuchera sanguinea coral bells

Spring Ephemerals

Scientic name Common Name
Dodecatheon meadia shooting Star
Erythronium albidum         white trout lily
Hepatica acutiloba sharp-lobed hepatica
Mertensia virginica Virginia bluebell
Sanguinaria canadensis bloodroot



References:

Harris, Richard W. and James R. Clark and Nelda P. Metheny. 1999. Arboriculture, Prentice hall, Upper Saddle River, NJ.

Still, Steven M. 1994. Manual of Herbaceous Ornamental Plants, Stipes Publishing Co., Champaign, IL.

Brown, Deborah L. 1997. "Gardening in the Shade", U of MN Extension Service, FS-1428-G0. http://www.extension.umn.edu/distribution/horticulture/DG1428.html

Hols, Marge. 1996/97. "Blooming Boulevards", U of MN Extension Service, Info U Script #467. http://www.extension.umn.edu/info-u/plants/BG467.html

Meyer, Mary H. and Michael E. Zins. 1998. "Ground Covers for Rough Sites", U of MN Extension Service, FS-1114-G0. http://www.extension.umn.edu/distribution/horticulture/DG1114.html.

Joyce, Mary S. 1996/97. "Perennials for the Shade", U of MN Extension Service, Info U Script #428. http://www.extension.umn.edu/distribution/horticulture/DG7566.html


This implementation report was developed by Kathy Ripke, student, University of Minnesota Department of Horticultural Science.

 
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