Heat’s Toll on the Garden

July 26th, 2007

   Don’t automatically reach for the garden hose just because your leaves are wilting.

   It may not be dry soil.

   It could be flat-out hot temperatures, which we’re likely to see more of shortly as we head into the blast furnace known as August.

   Heat doesn’t get nearly the blame for summertime plant woes as drought.

   That’s because heat is a more insidious and less understood plant menace.

   The late Washington plant pathologist Dr. Marc Cathey, in his 1998 book “Heat-Zone Gardening” (Time-Life Books), said that all kinds of trouble starts at 86 degrees:

   * That kind of heat causes the flower buds of many plants to wither.

   * It begins to shut down chlorophyll production, robbing leaves of their healthy green color.

   * It causes pollen to become non-viable, preventing popular plants such as tomatoes from setting new fruit until the weather cools.

   * It causes subtle chemical changes in plant leaves, rendering them more vulnerable to bug attack.

   * Especially if you don’t have mulch over the soil, it heats up soil temperatures to the point where root activity slows and plant growth is stunted.

   * And most noticeable, it jacks up moisture loss from plant leaves, making plants more susceptible to dry soil.

   Harrisburg is certainly no stranger to summer temperatures of more than 86 degrees.

   In an average summer, we get 30 to 45 days in that range. Given the climate-warming trend, we’re probably looking at more as time goes along.

   So what should we do, assuming air-conditioning the yard is out of the question?

   Rethinking our plant selection is one option.

   Like people, some plants take the heat much better than others.

   That was the gist of Cathey’s book, which broke new ground by rating plants based on their heat toughness.

   The book assigned numbers to what a lot of astute gardeners already have observed, for example, that annual flowers such as vinca, celosia and zinnia thrive in August steam while lobelia, osteospermum and nemesia peter out.

   For awhile, there was talk of adding these ratings routinely to plant labels.

   But for some reason, the whole idea just hasn’t caught on. You’ll have to get a copy of the “Heat-Zone Gardening” book to see all the ratings.

   A second anti-heat factor is plant siting – exactly where you plant what.

   Just about every yard has different microclimates. It might be hot and brutal out in the middle of the back yard or on the west side of a brick wall but 10 degrees cooler along the eastern foundation or under a shade tree.

   By matching a plant to its preferred heat and sunlight tolerance, you hold the power to make a plant thrive vs. frying it to death.

   If you guess wrong, don’t be afraid to move the plant to happier quarters… the sooner the better (just not in the heat of summer).

   A third issue is keeping plants as healthy as possible with good soil and adequate water.

   Plants lovingly planted in rich, loose, composted-enriched soil are going to put out better roots than ones jammed into lousy clay or packed shale. And that makes them better able to deal with any stress, including heat and drought.

   When it comes to water, don’t overdo it.

   I’m not sure I believe it, but some claim that even in a drought, more plants die because of too much water than not enough.

   Nonetheless, just because it’s hot doesn’t mean your plants need more water.

This hydrangea has wilted from intense heat, not lack of water.

   The best example is the old-fashioned hydrangea bush.

   This plant is notorious for wilting in the afternoon heat – especially when they’re planted in full sun.

   Hydrangeas with adequate soil moisture will quickly rebound overnight and look normal the next morning. However, a lot of gardeners see the late-day wilting, assume the soil is dry and soak the heck out of the ground around them.

   This may go on day after day until the plants die, and the gardener concludes they were just too far gone to revive. In reality, the roots rotted from being drowned on a daily basis.

   The moral: Check the soil first to determine if it’s dry enough to need more water. One of the best indicators is a simple index finger stuck into the ground.

   By the way, plants that are still wilted first thing in the morning probably do need water – or they’re in the process of dying from disease or other trouble.

   A rain gauge also will help you keep track of recent rainfall on your property. This is especially helpful since summer storms can routinely dump on one area while missing a neighboring area (which usually includes your house) altogether.

   A good covering of mulch over the beds is another huge help. A 2- to 3-inch layer of shredded bark and/or chopped leaves not only is great for holding down weeds and slowing moisture loss, it’s highly effective at keeping roots cool.

   Bare soil in the sun can easily be 20 or more degrees hotter than the air temperature, and on 90-degree days, that’s very bad news for plant roots.

   The ideal time to mulch is in spring after the soil has warmed and the early round of weeds has been pulled. But even mid-summer is better than letting unprotected ground bake for another six weeks.

   Long-range, planting more trees is a great idea. More shade means lower temperatures – for your plants, for your house and for you.

   Trees cool beyond just producing shade. Their leaves also cool surrounding air by transpiring moisture. It’s no coincidence that air temperatures near wooded areas is cooler.

   If you’re not that patient, erect a few arbors or pergolas and plant them with vines.

   Or plant a line of evergreens or erect fencing to block hot summer winds.

   Or move north. Iceland should be safe.

   Some plants that really don’t like it hot:

   Cedar

   Delphinium

   Enkianthus

   Fir

   Forget-me-nots

   Heather

   Hemlock

   Lady’s mantle

   Lychnis

   Marsh marigolds

   Primrose

   Pussy toes

   Spruce

   Verbascum

   Virginia bluebells

   Yew

   Source: “Heat-Zone Gardening” (Time-Life Books, 1998)

   Some of the most heat-tolerant annuals for Harrisburg-area gardens:

   Ageratum

   Begonias

   Cannas

   Cleome

   Cosmos

   Geraniums (Pelargonium)

   Heliotrope

   Marigolds

   Melampodium

   Nasturtium

   Nicotiana

   Portulaca

   Salvia

   Sunflowers

   Verbena

   Vinca

   Zinnia

   Some of the most heat-tolerant perennial flowers for Harrisburg-area gardens:

   Agastache

   Artemisia

   Black-eyed susans

   Boltonia

   Catmint

   Cimicifuga

   Coralbells

   Coreopsis

   Daylilies

   Gaillardia

   Garden phlox

   Gaura

   Goldenrod

   Hardy geraniums

   Hollyhocks

   Lavender

   Liatris

   Mums

   Penstemon

   Purple coneflower

   Salvia

   Spiderwort

   Some of the most heat-tolerant trees and shrubs for Harrisburg-area gardens:

   Abelia

   Aralia

   Arborvitae

   Bald cypress

   Barberry

   Beautyberry

   Beech

   Birch

   Boxwood

   Burning bush

   Butterfly bush

   Caryopteris

   Chokeberry

   Clematis

   Crape myrtle

   Daphne

   Dogwood

   Flowering pear

   Fothergilla

   Hawthorn

   Holly

   Honeylocust

   Juniper

   Oak

   Magnolia

   Maple

   Pieris

   Pine

   Redbud

   Rhododendron

   Shrub rose

   Smoketree

   Spirea

   Summersweet

   St. Johnswort

   Sweetbox

   Quince

   Virginia sweetspire

   Vitex

   Witch hazel

   Yellowwood