Monitor garden's water needs in hot, dry weather

If hot and dry weather persists, monitor your garden's water needs. How much you should water will depend on a variety of factors such as the soil, types of plants, weather, and exposure (sun and shade).

Plants that need water will show symptoms such as wilting, changing color and dropping leaves. To check the soil for dryness, dig down 3 to 5 inches.

In general, water thoroughly and deeply each time to encourage roots to grow deeper, rather that lightly and more frequently. I prefer watering in the morning to allow plant foliage to dry out before nightfall. Damp foliage can invite fungus diseases.

Trees and shrubs

Prune shrubs that have put on a lot of growth this year to keep them in the proper scale. The new growth will be mostly hardened off in early July, so pruning at this time should not stimulate much new growth.

Try using a pair of hand pruners instead of an electric hedge clipper. For an informal look, cut branches individually at different heights throughout the shrub. For a more formal look, make cuts at the same height. Prune right above the leaves to help hide the cuts. Careful pruning will leave the plants smaller without looking “sheared.” Also remove dead wood as necessary.

This is the time of year to check magnolias closely for magnolia scale, which is a flat round insect about the size of a pencil eraser. Typically found on the underside of magnolia branches, the scale eats the sap and can make the tree thin and unhealthy. Often the most noticeable sign of magnolia scale is a black sooty mold. The mold itself is not a real problem, but it can indicate a magnolia scale infestation because it forms on the clear sticky honeydew that the insects exude after feeding on the sap.

On small trees, simply pick off the scale insects and squash them. Other control measures can include spraying with summer-weight oil or applying an insecticide when the insects are in the active crawler stage, generally in early September. The timing is important because unless the scale insects will not be affected unless they are at the vulnerable crawler stage of their life cycle.

For trees with a history of magnolia scale, a late winter to early spring application of dormant oil will provide good control for the scale. If your tree has magnolia scale, provide extra water during summer to minimize stress.

Prune out water sprouts (vigorous shoots on the inside of the tree and on the trunk) and suckers (vigorous shoots growing from the base of the plant). Crabapples and hawthorns tend to send out lots of water sprouts and can benefit from this type of pruning.

Espaliered trees or shrubs are used for softening large blank spaces on walls or fences. To train espaliers, bend nearby branches gently into the design when they are young and supple. Carefully tie the branches in place with raffia or plastic tape. The ties should be secured loosely so the flow of water and nutrients through the stem is not constricted. Make adjustments to ties during the remainder of the growing season as necessary.

Emerald ash borer is becoming more prevalent in Chicagoland. This insect attacks ash trees and will kill them in time. Professional treatment with a specific insecticide may deter the insects from invading a tree. If your ash trees are not yet infested and you want to try to save them, contact an arborist about a to begin a treatment program. Treatments will be required every one to two years depending on the product you use.

Treatment will not save a tree that is already infested. If you have an infested ash tree, replace it with a different type of shade tree.

Some varieties of crabapple are very susceptible to a fungal disease called apple scab, and those not treated in spring may have lost many of their leaves by late July. The remaining leaves will develop black and yellow spots. Spraying with a fungicide at this point will not help, but give the affected tree extra care by providing supplemental water as needed and fertilizing in fall or next spring. Consider replacing disease-prone trees with resistant varieties.

General garden care

Monitor vines such as clematis and train them to their supports. It is best to do this on a regular basis to direct the vines where you want them to go. Once they have been allowed to grow out for a few weeks without support, it will be difficult to train them back up.

This is a good time to cut some of your outdoor flowers for indoor display. It is best to cut flowers during the cool morning or evening and to put them in water as quickly as possible. Remove any foliage that will be under the water in the vase. Keep your arrangements in a cool room out of direct sunlight. Adding a preservative to the water can lengthen the life of the flowers.

If your hybrid roses have been losing their lower leaves and the remaining leaves have yellowish foliage with dark spots, it is likely you have black spot, a common fungal disease. Begin a spray program with approved fungicides immediately. The fungicides need to be applied once every seven to 10 days, as they work to prevent the disease and do not cure plant parts that already are infected.

Be sure to clean up any leaves that have fallen from the plants, as they can spread the infection. Many shrub roses are resistant to black spot and do not need to be on a spray program.

Common rust of roses is not usually a problem in Illinois, but the early spring weather created optimal conditions for this disease to develop. You would have seen orange-red spots on the upper surfaces of leaves in early spring, but the disease is most conspicuous and serious in its summer stage.

Look for dusty orange spores on the lower surfaces of the leaves and an overall rust-colored appearance to the leaves. These spores will turn black and infected leaves may fall early. Remove infected leaves and spray the plant with a fungicide every seven to 10 days. Make sure the fungicide is labeled to control both rust and black spot.

If you want to maximize the size of your dahlia flowers, keep only the top flower on a stem and remove all side shoots so the plant directs its energy into the single flower. This will be most effective when growing the large dinner-plate sized cultivars. The plants likely will need some support to keep stems from breaking in the wind or from the weight of the enlarged flowers.

Weeds growing between cracks in brickwork or sidewalks are unsightly but easy to eliminate. Treat them with a nonselective herbicide when they are small to avoid having to pull them out by hand.

Bearded iris can be divided and replanted after they have finished blooming. Be sure to discard any shriveled or diseased parts. Be careful not to plant the new sections too deeply.

Vegetable gardens

Keep in mind these general rules when harvesting most vegetable crops:

Harvest vegetables when they are at the peak of their flavor. Younger plants and fruits are often more tender than those left on the plant longer. The length of time vegetables remain edible depends on weather conditions. High temperatures hasten maturity.

Handle vegetables carefully during harvest time. Check the garden daily and remove any ripe, damaged or misshapen fruits. Fruits that are not easily removed from the plant, such as eggplant, should be cut with a knife.

Harvest on a regular basis to encourage production. Many plants, such as cucumber, okra and zucchini, will cease production if mature fruits are not harvested.

Lawns

Established bluegrass lawns need the equivalent of 1 inch of rainfall a week to continue to actively grow and stay green throughout the summer. Otherwise, the lawn will go brown and turn dormant. It will revive and become green again when autumn brings cooler temperatures and more rain.

If you want to keep your lawn green by watering, water deeply once a week rather than lightly multiple times a week. This promotes a deeper root system so the grass will hold up better to stress. If your soils are heavy and water starts to run off after long periods of watering, you may need to split the watering into a couple of sessions per week.

To determine how long it takes your sprinkler to deliver 1 inch of water, set out a coffee, tuna or other shallow can with straight sides and time how long it takes to fill with an inch of water.

Be consistent with your watering practices: Either water on a regular basis all summer or let your grass go dormant during hot, dry periods. Mowing your lawn at a height of 3 inches or more will help the grass withstand stress and keep out weeds.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.