Jill Mulligan has worked in garden design and consulting.

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Overcoming Drainage Problems
by Jill Mulligan    

Have you lost any silver-leafed lavenders or ‘Silver Brocade’ artemisia or had tulip bulbs or Ruta graveolens ‘Blue Beauty’ just die, often after only one winter? You may be wondering why. Many plants benefit from “well drained” or “evenly moist” soils.

According to Jacqueline Heriteau, editor of The American Horticultural Society Flower Finder, “Evenly moist soil can be found next to a running stream, or it can simply be well-drained soil with enough spongy humus in it to be water retentive. Boggy soil is consistently wet but not necessarily well drained.


Soil Texture

All our gardening successes come down to the microscopic size differences between soil particles because these determine water retention/ drainage and air availability. Ideally, soil is 50 percent solids, organic matter and mineral particles – sand, silt and clay – and 50 percent pore spaces that hold air and water.

You can add organic matter in the soil to improve its friability (the way the particles clump together with pore spaces for the air and water between them). Improved soil can absorb more water and reduce erosion and loss of topsoil.

The more soil that is oxygenated, the better roots can grow. When we walk or drive equipment over the soil, we are collapsing the oxygen pore spaces and compacting it.

Clay soils contain many tiny particles that stick together and retain moisture. Coarser sand particles allow water to drain more rapidly. “With too much water, plants will suffocate,” said Ralph Hall, a Virginia Cooperative Extension Master Gardener. “With too little, plants will eventually die.”

The balance of air and water in good soil structure allows for optimal root growth.

With wet soil and not enough air, for instance, bulbs will start to rot and decompose during the winter. Organic matter can improve soil structure immediately, with continued benefit over the growing seasons. Best of all, you can add your own compost as well as composted leaf mulch into your planting beds by mixing them into the soil or using them as mulch. You will need to continue adding organic matter to your garden to improve and maintain your soil to its “well-drained” best. It’s never too late to improve the texture and structure of your garden soil.


Design a Stress-Free Garden

Yellow weeping willow is not a native, but it loves wet feet.

Good garden design should take soil drainage into account to keep your plants flourishing and beautiful. Stressed plants don’t thrive. By placing your plants in the best locations for their moisture requirements, you will reduce one stress that causes disease and pest problems as well as lowering plant maintenance.

Site your plants so that water lovers are in depressions or low spots of your garden. The soil on slopes is not necessarily well drained unless the soil has been amended with organic matter. The soil at the bottom of the slope will be wetter, so beware of mixing “wet feet” lovers with plants that prefer dry soils.


Native Trees And Shrubs For Moist Soil

Amelanchier, also known as shadblow, is a multi-stemmed tree that grows 10 to 25 feet tall. The smooth, gray bark, small refined leaves, reddish buds and white flowers that bloom in April to May contribute to its four-season interest. It has beautiful yellow to red fall color, and birds love its bluish-black berries. Grow it at the edge of woods or in a naturalistic setting with coarser ground covers such as hosta or pachysandra. Many gardeners are substituting Amelanchier arborea for flowering dogwood as an understory tree in a naturalistic setting. It is hardy in Zones 4 through 9.

‘Heritage’ river birch is a fast-growing variety of Betula nigra with creamy white, exfoliating bark. It is heat tolerant in Zones 5 through 9 and resists birch borer. It tolerates shade and needs an acid soil pH of 6.5 or less in order to thrive. Its roots seek water, so don’t plant birch trees near water mains or septic systems. ‘Heritage’ has a rosier bark than another native, the paper birch (Betula papyrifera), which looks beautiful against evergreens, in large areas and grows best in Zones 2 to 6.

White fringe tree (Chionanthus virginicus) is a multi-stemmed tree or shrub that is found along stream banks. Its fleecy, soft textured, fragrant white flowers bloom in May or June. It grows 12 to 20 feet tall and looks lovely as a specimen shrub. It is tolerant of air pollution and is hardy in Zones 3 through 9.

A Styer Award winner, ‘Blue Mist’ dwarf fothergilla (Fothergilla gardenii ‘Blue Mist’) is a small shrub 3 feet tall that has blue-green summer foliage. With fragrant white flowers that look like bottlebrushes, it can be massed with azaleas and other rhododendrons in foundation plantings or used as an understory shrub. It is also pest and disease resistant and likes sandy loam and full sun for best bloom.

Sweetbay magnolia has lemon-scented, ivory flowers in May and June.

For a lemon-scented shrub that likes wet feet and tolerates shade, you can use sweetbay magnolia (Magnolia virginiana). Its fragrance, small scale and smooth, grayish white bark provide nice features for use near an entrance or patio. It blooms for six weeks in May to June, late enough to avoid the late frosts that can bedevil star magnolias. Sweetbay magnolia is hardy in Zones 5 to 9.

Southern magnolia (Magnolia grandiflora) is a noble tree. Use it as a large specimen or in mixed stands with tulip poplars and sweet gums. Southern magnolia has a coarse texture that contrasts well with finer textured trees such as mugo pines or hemlocks. It can grow 60 to 80 feet high and 35 to 50 feet tall at full maturity. However, it grows about 12 inches a year, so it is considered a slow grower. Be sure to give it lots of space since its roots don’t necessarily stop at the drip line. It is hardy in Zones 7 through 9.

Mountain laurel (Kalmia latifolia) should be sited near streams or riverbanks, in cool, moist, acid, well-drained soil. It flowers best in full sun but will also grow in heavy shade. Mountain laurel has fibrous roots that don’t compete well with thirsty trees, so consider using it in large containers as well as in naturalistic settings. It is hardy in Zones 4 through 9.

You can increase the charm and survivability of your plants by growing them in the right locations.


(From Virginia Gardener Volume II Issue V. Photos by Jill Mulligan.)



Posted: 01/18/12   RSS | Print


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