the show

Waterwise Plants for Sustainable Gardens

encore date: May 26, 2012

original air date: April 14, 2012

Sign on to water wise plants for sustainable gardens with horticulturists and authors, Lauren Springer Ogden and Scott Ogden. On tour, see how Anne Bellomy dumped lawn in favor of wildlife plants. Daphne Richards answers: “Can I put soil or compost over exposed oak tree roots to plant groundcovers?” Her pick of the week: drought-tough native Mountain Laurel. Merrideth Jiles from The Great Outdoors shows off his home garden survivors that made it through 2011’s hard freeze and drought.

Question of the Week

Can I cover my oak tree roots with compost or soil to plant groundcovers?

Older trees can have large exposed roots. They can actually be a hazard, so if you can cover them, you definitely should. And if the tree is in the lawn, the roots are also in the way of the mower and can get nicked, which wouldn’t be good. Building up the soil over those roots and planting a groundcover, one that doesn’t need to be mowed, is a great idea. Just make sure that the soil/compost or mulch that you add doesn’t touch the trunk, and that you don’t apply it too thick. Two to four inches is a good amount, but don’t apply more than that-you don’t want to cover those roots too deeply.

I’m sure you’ve noticed the bark-like texture of the exposed roots. Although they once took up water and nutrients for the tree, now they serve as support, and are connected to the feeding roots, which are found at the drip line of the tree, out past the furthest branches.

If you do plant a groundcover over the exposed roots, be careful not to damage the roots when you dig around them. A wounded spot will allow for fungi and bacteria to invade, which could damage your tree. And a large cut might result in die-back in the part of the tree connected to the cut root.

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Plant of the Week

Mountain Laurel

Mountain Laurel

Sophora secundiflora

This native shrub/small tree is an evergreen that is extremely drought tough. Even in years of extreme drought, they survive when other natives have died. It is a slow grower, but well worth the wait. It's an excellent screening or accent tree. Height at maturity: 10-20 feet tall; 8 to 12 feet wide Light: full sun is best but can take part shade. Soil: Adapts to most soil types but wants good drainage. Do not over water once it is established. It has a very long tap root, which makes it drought-tough (but also harder to move!). Flowers: Large scented flowers in early spring that attract all kinds of beneficial insects. Many people describe the fragrance as grape Kool-Aid! Why not bloom: Maturity is one reason. The other: pruning off flower spikes, which form very quickly in spring, just after this year's flowers. These knobby growths may look odd, but they represent next year's flowers. Mountain laurels should only be pruned at trunk branches to shape (if necessary). Also, Texas mountain laurel can be attacked by the Genista caterpillar. In one day, they can defoliate a tree, so be sure to apply Bacillus thuringiensis (Bt) at the first outbreak. Most likely, you will only have to spray once, but in intense invasions, you may not need to reapply. This foliar application will disrupt the caterpillars' digestive system and they will stop feeding and die in a few days. Deer resistance: Yes. Seeds: The seeds are poisonous if swallowed, but not dangerous otherwise. Once fully mature, the seed pods turn dark brown or gray, and the seeds inside are dark red. The seeds have a very heavy seed coat, making them hard to germinate. But if you wish to try, it's best to harvest the seed pods before they are fully developed and plant the seeds before they have turned red.