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Trees aren't always the best companions for plants

When gardeners see large expanses of unoccupied soil under mature trees, they are often seized with the urge to fill in the space with flowers or vines. However, this can cause some problems.

While we may imagine that trees have a large taproot, the truth is that most of them are quite shallow-rooted, with roots being in the top 12 to 18 inches of soil, and spreading beyond the edge of the canopy, or drip line. Worse, many trees have roots that are exposed at the surface, quickly crowding plants that may grow there and hogging any available moisture.

Some of the worst offenders are birch, beech, sweetgum, locust, willows and most maples.

Evergreens such as western red cedar and Douglas fir present another problem. Their dense foliage is so efficient at shielding the ground from winter rainfall that the soil under them is extremely dry all year. If you try to grow plants under those trees, it will take careful planning and extra effort.

Another set of problems can arise if you try to grow plants under two of our natives, madrone and Oregon white oak. They will not tolerate being given summer water, to the point that they may turn up their toes and die as you are busy watering the plants you've put under them.

Still determined to plant under your trees? If so, here are a few tips.

  • Choose plants with shallow roots, as they do better at competing with tree roots. Try sedum, salal, adjuga and lily of the valley. If you choose perennials, select ones that do not need frequent division, such as anenome, hellebores, trillium and hosta.
  • Under trees mentioned above that do not like summer water, plant epimedium (bishop's hat), hardy cyclamen, spring bulbs, sword fern, Oregon grape, vinca or kinnikinick.
  • Dig individual holes for your new plants, instead of preparing a whole bed at once. With proper care, a plant from a 4-inch pot will establish quickly despite competition from tree roots.
  • Add plants under established trees over several growing seasons instead of all at once.
  • Do not plant closer than two feet from the tree trunk.
  • Small tree roots may be cut if needed; avoid cutting any that are thicker than 2 inches in diameter.
  • During the first year, give supplemental water to the new plants to help get them established.
  • Use a light hand with mulch. It should not touch tree trunks or be heaped around plants.
  • Never use weed-and-feed around trees. It will slowly but surely kill them.

If the above ideas do not fit your circumstances, you might create a shady, leafy retreat with a bench, shade-loving plants in containers, and maybe some garden art.

Coming up: Kelly Brainard of Ashland Greenhouses will discuss Structures to Extend the Growing Season, from 7 to 9 p.m. on Tuesday, Sept. 9, at the Southern Oregon Research and Extension Center, 569 Hanley Road in Central Point. Brainard will provide information on construction of greenhouses, hoop houses and cold frames. Call 541-776-7371 to register. The cost is $10.

Carol Oneal is a past president of the OSU Jackson County Master Gardeners Association. Email her at diggit1225@gmail.com.