Gardening Help FAQs

Here are answers to some of the most common questions we receive about garden plants. You will find concise information on general gardening techniques as well as plant selection and care. For detailed information on specific plant pests and problems refer to our Common Garden Pests and Problems page.

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Horticulture Questions and Answers

What problems can wet weather cause to my plants?

Extended periods of wet weather with persistent rains may be the cause of some plant problems even though plants may not be flooded. Although there may not be any visual damage at this time, a tree or shrub may have experienced root damage, disease or other damage during the very wet period. Most often there is suppression of the root system. After the wet weather has subsided, assess all plant growth and examine for any weak or reduced growth. If there is an indication that a plant may have been weakened, some special summer care may be needed.

One of the most common reasons for the suppression of plant growth during very wet periods is the reduction of oxygen in the soil that is available to plant roots. Plant cells respire and need oxygen to survive and grow. While we breathe oxygen from the air that is then distributed throughout our body, plant cells must get oxygen from the air that is close to the cells. Oxygen of the air in the soil is not as high as oxygen of the air around us. If the soil air drops to less than 12% oxygen, roots will suffocate and die. Excess water in the soil forces out soil air. When there is less air available, the oxygen is rapidly used and becomes inadequate for good root growth. Also, in very wet soils organic matter begins to decompose and this process also further depletes oxygen in the soil environment.

Plants in standing water or highly saturated soils will lose some of their deeper roots first, but as the condition persists, shallower roots may also be damaged. The greater the number of roots killed, particularly the small feeder roots, the greater the stress on the plant after conditions dry. Plants may maintain abundant leaf cover during wet periods, but if the root system has become reduced, under dry or hot weather wilting, leaf scorch and die back may develop. Newly planted trees and shrubs may have been unable to produce abundant new roots as would be possible under more normal conditions.

Water uptake in plants decreases within an hour after the soil oxygen drops below the critical level. Consequently, a plant will wilt even though there is moisture than enough water in the soil. If newly planted trees or shrubs are subjected to this condition, the new roots that help them get established after transplanting will become oxygen starved and die almost as soon as they are formed.

Plants that have lost roots during wet periods are not well prepared to face summer heat or drought. If the summer remains fairly moist and cool, few problems may be noticed as plants gradually recover. However, if weather becomes very severe, many trees and shrubs will show damage or may even be killed. These same conditions may develop if plants are overwatered after transplanting even though soil moisture naturally may not be excessive.

Watch plants carefully during the coming months, particularly those that have been planted this year or during the past few years. Water them promptly during hot or dry periods. Normally, an inch of water per week during normal summer weather either as rainfall or irrigation is adequate. Water well, but avoid frequent light watering or adding excessive amounts at one time.

In addition to root problems, diseases often get established on leaves or roots of plants during extended wet periods. Sycamore anthracnose has been serious in many areas. Black spot of roses has also become well established in some gardens. Grass in the lawn is also subject to certain diseases that could have become established during the frequent rains. These problems are good reasons for close observation of landscape plants and lawns during this month. If any dead spots, die-back, leaf spots or other disease problems become apparent, whatever the plant, prompt action is important to prevent more severe damage. A fungicide approved for use on the damaged plant should be used. For these types of diseases, liquid sprays are generally more effective than dusts. In some cases, such as apple scab on flowering crab apples, it is too late for control measures when disease damage can be seen.

In the garden or flower bed, particularly where there is clay soil and no mulch, a crust may form on top of the soil. Break up the crust to provide better soil aeration when conditions are dry enough. To avoid a repeat problem after the next heavy rain, a mulch will keep the surface softer and allow better air exchange into the soil. It will also keep down weeds that can get a head start when soils are too wet to work.

By Ray R, Rothenberger, Garden Spotlight, University of Missouri Extension