Sometimes leafy plants, like this squash, wilt because the air is just too hot and dry, not for lack of water in the soil. (Daily Pilot / August 27, 2010)

For better or worse, summer heat has finally descended upon our gardens.

Certainly, local gardeners will experience more periodic heat spells during the next two or three months, so how should a local gardener respond to these heat spells? With a cold glass of lemonade? Well yes, but what about our gardens?

Because a plant's reaction to heat is gradual, its impact is often misunderstood. In a futile attempt to mitigate heat stress, a gardener's usual response may only be to water more. But in many cases, more water won't make things better. In fact, it could only make things worse, promoting root diseases, which thrive in hot, moist, compacted summer soils.

Here are a few tips:

•Strong, well-rooted plants withstand heat better than weak plants or plants with small root systems.

•Check the soil. It may be dry on top, but moist a few inches below, where the roots are. Don't necessarily water because the soil surface looks dry.

•Make sure thirsty plants are well-watered before extreme temperatures hit. Usually that means watering in the morning or evening. Several plants will wilt during the hottest part of the day, no matter how wet the soil is. These plants are simply losing water through their leaves faster than their roots can replace it.

•A cool spritz of water on the leaves, not on the soil, will slow down a plant's water loss and perk the leaves back up, using surprisingly little water.

•If using a hose, be sure to let it run for a minute or two, until cool water comes out.

•Mulch, mulch, mulch. It keeps the soil from heating and drying out and conserves water.

•Many established California native plants can survive with little or no summer irrigation. They may enter a stage of "evergreen dormancy" while they hang on through the hot dry months. More water won't help and may actually harm them. Use restraint.

•Avoid pruning most plants during the summer. Removing too much leafy growth will expose the stems and branches to strong sunlight that can cause sun-scald.

Heat damage may first appear as leaves that droop, flower buds that wither and drop, plants that stop growing or leaves that shrink. Less visible to the gardener are the roots of their plants, some of which may stop growing altogether in hot weather. Many Mediterranean plants, including most of our California native plants, even retract their roots as a response to hot weather. In an annual cycle, the roots of these well-adapted plants expand in the cool, moist winter months and shrink during the hot, dry summer. Because plant stress from heat is slow and lingering, it is often obscured by secondary afflictions, like the root rots mentioned above and summer pest attacks. But most often the plant survives, it just looks unhappy.

If you're the scientific type, read on. Plants breathe through tiny openings on the undersides of their leaves called stomata that open and close at different times during the day. In a plant, this breathing process is called transpiration.

Plants do not have lungs, they have stomata. When open, carbon dioxide enters a leaf through these tiny little pores, while water simultaneously exits the leaf. As water is out through its leaves, the relative humidity is raised around the plant and the surface of the leaf is cooled. This water loss, created when the stomata are open causes a surface tension among the water molecules, tending to pull the next one along behind it, up through the plant's veins. Water is ultimately pulled all the way from a plant's roots, up to its leaves and out through its stomata. This upward pull of water creates negative pressure in the root zone, allowing roots to literally suck moisture from the soil and into the plant.

Stomata are the regulators of the entire water movement process in a plant. They open and close, dependant upon certain conditions, especially heat. For example, if heat becomes excessive and causes a plant to lose more water than it can take up, the plant closes its stomata, slowing down its water loss. Unfortunately, closed stomata also slows down the plant's cooling mechanism. This causes heat to build up in the plant. Several days of this and a walk through the garden may reveal a lot of heat-stressed plants.

But that's not all. When stomata are closed, a plant is not pulling water from its roots, up through its veins and into its leaves, which further contributes to the "stressed" appearance of the plants. Without water transpiring through open stomata, nothing is being drawn up from the root zone, including nutrients. Without nutrients growth is slowed and flowers and fruit may even fall from the plant, a common complaint of those who grow summer tomatoes.

By understanding how water is used in plants, you will be able to use it more wisely. Often, more water applied to the roots may not help. Maybe just a spritz is all the plant needs.

RON VANDERHOFF is the nursery manager at Roger's Gardens in Corona del Mar.

Ask Ron

Question: Last year I went to the OC Master Gardener fall Gardening Seminar in Huntington Beach. When is it this year?

Jack

Huntington Beach

Answer: The eighth annual OC Master Gardener Gardening Seminar will be from 9 a.m. to 3 p.m. Sept. 25 at the Huntington Beach Central Library. More information and registration is available at http://www.uccemg.com. It may be the best six hours of gardening training available in Orange County. Don't miss it.

ASK RON your toughest gardening questions, and the expert nursery staff at Roger's Gardens will come up with an answer. Please include your name, phone number and city, and limit queries to 30 words or fewer. E-mail stumpthegardener@rogersgardens.com, or write to Plant Talk at Roger's Gardens, 2301 San Joaquin Hills Road, Corona del Mar, CA 92625.