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Pests don't take summer off

Garden insects and root rot can undermine plantings weakened by improper watering. Now's the time to take action to protect your yard.

August 07, 2003|Lili Singer | Special to The Times

By midsummer, few landscape plants and garden pests are terribly active. Even so, good gardeners know not to rest. Before and after the heat of day, they devote themselves to fortifying their gardens against hot weather and the spoilers that prey on weakened plants.

No matter what your garden's size, content or style, preventing drought stress in new and existing plantings is summer's most important garden task.

Inadequate irrigation leaves plants vulnerable to many pests, some potentially deadly. Phytophthora fungi that cause root, stem and crown rot can sense chemicals released by the roots of a drought-stressed plant; when water becomes available, they head straight for that weakling. Female boring insects seek out and lay eggs beneath the bark of trees whose sap has been diminished by drought; they know their larvae won't drown there. Once inside either a neglected backyard plum or the frail pines in local forests, borers are virtually impossible to control.

Knowing when to replenish water to a plant's root zone at just the right intervals takes experience and some sense. Most gardens need more irrigation in summer than in cooler months, but don't change your watering schedule blindly. Monitor the weather, check soil moisture frequently, then irrigate deeply and thoroughly, but only as needed.

In summer and in general, the soil should dry out slightly between waterings: 4 to 6 inches down for mature trees, shrubs, vines and ground covers; 2 to 4 inches for perennials; and 1 to 2 inches for turf grass and shallow-rooted annuals.

A sturdy steel soil probe will help you determine when to water. This nifty tool, also called a soil sampler, smoothly extracts a core of earth, revealing soil moisture, composition, root health and more. To take a sample, push the tube straight down into the soil as far as possible, twist one-quarter turn, and lift straight up.

If your local nursery doesn't stock soil probes, ask whether one can be special-ordered. Denman & Co., a garden tool store in Orange ([714] 639-8106), stocks one model. Online sources include Gempler's (, Peaceful Valley Farm Supply ( or Harmony Farm Supply


Only new transplants, seeded areas and just-laid sod need light, frequent watering. For the first few weeks after installation, they should be checked daily and sprinkled when the soil surface dries. After a few weeks, however, young roots are developing and it's important to switch plants to deeper, less frequent soaks.

Container gardens need special attention in summer, especially those in small pots or those in pots made of highly porous wood or unglazed clay. Use a chopstick to measure soil moisture and water when the top is dry. Cluster pots together; they'll insulate one another and dry out less quickly.

If any of your landscape plants look dull and weary, and you have determined they don't need water, give them a summer spritz. Refreshing rinses every week or two, especially during warm, windy periods, will remove dust along with insects, mites and their eggs. Be sure to spray the undersides of the leaves.

If a plant is wilting, especially new growth, and the soil is still moist, you have a problem. Examine a soil sample from the root zone. If there are no succulent white feeder roots and larger roots appear sickly, suspect root rot. Fungicides can help, but they'll do no good unless you also water far less often.

In summer, mulches confirm their remarkable ability to conserve moisture, insulate roots and retard weed growth. A 3- to 4-inch layer of organic matter, such as composted bark and wood fibers or, better yet, your own compost, works best.

As organic mulches decompose, the soil is enriched by nutrients and beneficial soil microorganisms. Researchers are sorting out just how composts and healthy soils subdue plant diseases, including Phytophthora.

To avert disease, keep all mulches at least a foot away from tree trunks and several inches from plant stems and crowns. Summer is a good time to restrain such "living mulches" as ground covers and understory shrubs.

Garden cleanup has aesthetic benefits; good sanitation is also the simplest way to break the cycles of reoccurring pests.

Pull and recycle summer annual weeds before they flower and set seed that will germinate in years to come. Aging weeds also act as "alternate hosts" for insects, mites and disease organisms that later attack desired plants in other seasons.

Some pests develop or take cover at soil level or on crops far past their prime. To manage brown rot, a disease of peaches and apricots, remove fallen fruit and desiccated fruit "mummies" left hanging on trees. Next, rake up and replace mulches. This nonchemical practice greatly reduces fungal spores and future damage.

Still-ripening edibles need protection. Use bricks or overturned pots to raise melons, squash and cucumbers off the damp earth and away from snails, slugs and other ground-level feeders. Enclose maturing grape clusters in netting or paper bags, making them slightly less accessible to birds. If such efforts prove futile, share your bounty with grace. Garden birds devour far more insects than fruit.

Postpone all major pruning. Delicate inner bark is easily sunburned, and sun-damaged wood invites borers and pathogens. As time allows, thin dead and crossing inner branches on California native trees and shrubs to make room for autumn growth spurts.

At high temperatures, usually low-risk horticultural oil sprays, including light mineral, vegetable-based and neem preparations, can be toxic to plants. If it's 85 degrees or above, play it safe and wait. In the interim, beneficial insects may find your intended target and manage those pests for you.

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