Wilting in the heat

It's well into the 90s in Chicago today, the hottest day of the year so far. This morning I passed a patch of impatiens in front of a church that were sadly wilting.

It's not like we're short on rain. But heat in itself is hard on plants. Physiological damage starts to set in at about 86 degrees, according to the American Horticultural Society. Plants that live in deserts have a variety of ways to survive hot, dry conditions, and we can recognize some of them in our garden plants: My lamb's ears and Salvia argentea, for example, have leaves covered with tiny hairs that reflect heat away from their tissue.

Such plants must have evolved in hot, dry places, so you know they aren't going to thrive in the same conditions that make ferns happy. I'm trying to keep some lamb's ears going in a part-shade place for the sake of their fuzzy contrasting texture. But I may end up transplanting them to the sunny bed at the rear.

Plants keep cool pretty much the same way humans do. In humans it's called sweating: we secrete water through our pores, which evaporates, reducing the temperature at the surface of our skin. In plants it's called transpiration. Plants let water out from the same openings in their leaves, called stoma, through which they take in carbon dioxide for photosynthesis. The water evaporates and cools the plant.

In some really hot, dry environments, water is precious, so plants have evolved to save up sunlight during the day and only open their stoma to collect CO2 at night, when it's cooler and less water will be lost.

Many plants have heat-beating strategies that involve conserving water. The fleshy leaves of succulents, with their waxy coating, are reservoirs. That's what makes sedums the favored choice for the blazing hot conditions of green roofs, where they soak up water in heavy rains and use it to keep themselves cool and operating through hot spells.

Chicago's type of heat is hard on a lot of plants for the same reason it's hard on me: The high humidity keeps water from evaporating readily. The combination of warmth and moisture also can encourage fungus diseases such as powdery mildew. That's why it's important to keep shrubs like lilacs pruned so air can circulate through them and give them a chance of drying off, and why it's a good idea to water only at the soil level and avoid getting water on leaves. (Not that watering is necessary so far this year.)

Impatiens walleriana, the brightly blooming annuals that are so popular in urban gardens and containers, are a forest-floor plant that originated in eastern Africa and are adapted to the shade. They don't have a highly efficient cooling mechanism, so they can't bear being out in the sun and they dry out fairly quickly. I regard them as an indicator plant: If I see impatiens starting to wilt I know it's time to water a lot of things that aren't yet stressed enough to wilt.

It's raining now, and it's supposed to cool off a bit tomorrow, which will bail out the impatiens by the church. But it's only June. Those impatiens are in a pretty sunny spot, and they're going to have a tough life in August.

Got a garden question? I recommend you call or e-mail the Plant Clinic of The Morton Arboretum in Lisle, the Master Gardeners of the University of Illinois Extension or the Plant Information Service of the Chicago Botanic Garden in Glencoe.

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