Regional Gardening Reports :: National Gardening Association

In the Garden:
October, 2010
Regional Report

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The Paulownia allee at Longwood Gardens - grassless area under tree canopy; no volcano mulch.

Less is More Under the Tree Canopy

"Everything I plant under my trees dies" is the most common complaint people have. "What can I plant there that will live?" is the most common question.

I shake my head. There IS a reason geraniums, petunias, phlox, even indomitable hybrid coral bells aren't likely to survive under a tree canopy. Well, really several reasons that make planting there a fool's errand. Common-sense, visible, obvious reasons, plus the invisible Big T - Transpiration.

On a recent visit to Longwood Gardens in Kennett Square to see the new Living Wall in the East Conservatory, I dawdled along the Paulownia tomentosa allee. The thick-girthed, pyramidal, big-leafed trees spread wide canopies along the walkway from the Visitors' Center. In spring, their lavender flower clusters are breathtaking. In summer and autumn, their heart-shaped leaves draw the playful eye.

Gray, mottled barked trunks push above knobby, thick root collars. The stuff of wicked imagination. Under each Royal Empress tree is a gray circle, about 15 feet in diameter, of weathered bark mulch. Protection. Non-competitive, biodegradable, water-permeable, attractive protection. The trunk is remote from any mower blade; the roots have full and singular access to moisture and soil nutrients.

Trees roots reach wide but aren't as deep as we might assume. Anyone who's seen the thick root snarl of an uprooted tree is likely surprised at the shallow root system. Most tree roots are within 18 inches of the soil surface. Roots do spread far and wide beyond the drip line (canopy), though.

Dense tree roots are one reason most perennials and annuals don't thrive under a tree's canopy. Too much competition for food; way too much competition for rain making its way through branches and leaves. Lack of sunlight limits food production, aka photosynthesis. Reality check - What CAN grow in a shady desert?

The Big T, transpiration, is a factor we frequently forget. Trees absorb water through their roots. That water finds its way into all, even the most tip-top leaves, then the atmosphere via the transpiration process. The average tree can transport 10,000 gallons of water from roots to leaves each year. The U.S. Geological Survey reports an oak tree can lift 40,000 gallons from roots to evaporation in a year.

What chance does a petunia or an impatiens have? Their meager roots are the losers in any competition with greedy tree roots. But some tough leafy perennials survive these arduous conditions, including hosta, epimedium, tiarella and native Heuchera. These aren't showy beauties that require sun and moisture to flower abundantly. They're mostly herbaceous, with small, short-lived flowers.

Despite the science - the reasonable explanations- people want what they want. They dig among the tree's feeder roots, damaging them, to plant flowers that require much hand watering to just survive. The question is, which is more important, more valuable- the tree or seasonal flowers?

Now when asked about planting under a tree, I can click on my Android and pull up a photo of Longwood's bare-bottomed Paulownias. If proper, level mulching under the drip line is good enough for Longwood, well, enough said.

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