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Gardeners cultivate questions about effects of severe heat on plants

By Joel M. Lerner
Saturday, August 21, 2010; E03

This summer has been a challenging one for keeping plants growing. Record-breaking temperatures in July and August have created heat stress for plants. According to the National Atmospheric and Oceanic Administration, the May-to-July period was the warmest on record for the Northeast and Southeast and the ninth-warmest for the Central region. This period produced record heat for the Washington area -- and a bumper crop of questions from area gardeners.

Q. You mentioned in a column that annual shearing was good for "emerald green" arborvitae. I have two, and in early spring I tied them gently to retrain the branches that had been pulled out of line by the heavy snow. After reading your column, I'm guessing I should not shear them until next spring, but what do you advise? How should I shear them? When does growth begin in spring? -- Dana and Ray Koch

A. If you have corrected the growth habit by tying the branches together and they are beginning to grow vertically, leave them tied together for the rest of the growing season and untie the rope before growth begins next spring. When I refer to shearing, that means using electric or manual shears. The foliage can also be selectively pruned to lighten the weight on the branches. Keep greenery wider on the bottom than the top so sun will reach the entire plant. Do this pruning in mid-March just before growth begins in spring.

Q. Yesterday I dug up three rosebushes and in the process had to dig up at least a hundred iris bulbs that were entangled in the roots. I cut the green leaves and left about 6 inches of roots. Can I replant the irises now or should I wait until spring? -- Barbara MacDonald

A. The irises might have matured a little faster had they been transplanted in July, but it's fine to transplant them now. Cut the fan of leaf blades to about half the original height and retain at least one healthy fan of leaves attached to a 4-to-6-inch-long root piece. The only viable roots, or rhizomes, are one-year-old pieces that should be firm without soft or rotten tissue. After they are groomed, plant rhizomes in soil that is about two parts native soil to one part compost, 1 to 3 inches deep. Firm the soil around the roots and allow the fans of leaves to protrude from the soil. Keep the area moist and well drained to encourage new root growth before winter.

Q. We had a row of Leyland cypresses shading ferns. The ferns died back every winter and came up in spring for the past 35 years. But we lost every Leyland with the weight of the snow in February. The ferns came up this spring and looked good, but the sun eventually took its toll. They are burned and look awful. Is there anything we can do to rescue them? -- Tahma Metz

A. Transplant them as soon as possible. The roots from which the fronds unfurl are drying out as you read this. Most ferns like moist shade and soil high in organic material. Soak the ferns before trying to dig them. When they're moist but not muddy, dig the clumps and keep them on the shady side of your house, or plant them now in a woodland area. If they didn't completely dry, some of the roots could still be viable. Get compost and mix it into the soil to keep it moist and improve drainage. Lay some shredded bark over the roots to protect the plants through winter. You might not know whether the ferns have survived until next spring. That's when the fiddles emerge and renew again into beautiful, healthy fronds.

Q. We're looking to replace some shrubs. I'd like something bushy or dense for privacy, so folks can't see over the fence into the back yard. The area has a northeastern exposure. I'm disabled, so I can't give the shrubs a lot of care. This space gets lots of sun. Any suggestions? -- Nancy Willis

A. If you have a fence for protection from late-day heat, most plants should thrive. Compost should be added to enrich the soil, and the new shrubs will need to be watered during dry periods. Judging from the height of most backyard fences, you'll want a shrub that will grow about 12 feet tall. The only other decision is choosing between an evergreen or a deciduous plant. Evergreens offer strong screening. Deciduous shrubs have flowers but not much winter screening. Several evergreens to consider are Wichita blue juniper, green giant arborvitae, robin holly, oak leaf holly and Chindo viburnum. Several deciduous shrubs to investigate are Chimonanthus praecox, Allegheny viburnum, hedge maple, common witchhazel and arrowwood viburnum. All plants need care, but these should require fairly low maintenance.

Q. My hydrangeas not only have sunburned leaves, but most of the flowers are burned as well. Should I deadhead the bushes and, if so, how far back can I cut the stems? I am wondering if deadheading will affect next years flowering. -- Pat Ludden

A. I am assuming that you are discussing a florist hydrangea (H. macrophylla). Keep the shrubs moist. Do not deadhead them or do any hard pruning unless a stem is dead. Deadheading and hard pruning will affect your plant's flowering next year because flowers form on this year's growth. If you are to have any flowers next year, you will have to tolerate their burned appearance until growth begins next spring.

Q. I have some yard issues. The first concerns English ivy and Virginia creeper. What can I do to get rid of these climbing vines that have taken over the yard? We're concerned about weed killers that might harm animals but want to get rid of the ivy even if it means using a strong weed killer. We also had three trees removed and the stumps keep pushing new growth. What can you recommend to kill everything and keep the wildlife safe? -- Steve Lease

A. First, remove the most overgrown vines by hand, digging and pulling them. Even if you spray herbicide, the key is regular attention when vines are young and less-toxic materials can be used. Vinegar works on very young plants and is safe. Use a plastic sprayer that won't corrode. Corn gluten meal will also keep weed seeds from germinating. BioWeed Herbicide by Certified Organics has a quick knock-down and is a fairly innocuous material. Every plant begins as a seedling, so keep areas in check by hand by using mulch or planting low-growing flora to minimize weeds. Wear gloves, long-sleeved shirts and pants in case you encounter poison ivy. Learn what vines you have at garden centers or plant clinics held by your local county extension. Some plants that need to be controlled in their seedling stage include: porceleinberry, wild grape, English ivy, Boston ivy, Virginia creeper and wisteria.

Use Green Light Cut Vine & Stump Killer to rot your tree stumps over a period of years. It contains triclopyr, a toxic material that is safer than some alternatives. It will kill the vegetation growing from the stumps, and then the stumps will rot. Be sure to follow label instructions.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md.

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