Face it: We live in a wet climate. Rain events that drop several inches of water are not all that uncommon. The recent heavy rains and street flooding certainly reminded us of this.
Our rainy climate has a profound effect on gardening in southeast Louisiana. It determines the plants we can grow in our landscapes and how we take care of them. Periods of heavy or frequent rainfall can have a negative effect on plants. Here’s how.
The upper parts of plants don’t really mind the rain all that much, although we do see an increase in the number of foliar fungal diseases during wet weather. This is because the spores of most fungi that infect leaves need a film of liquid water to sprout and cause an infection.
The more leaves stay dry, the less opportunity there is for fungal infections to occur. The more the leaves stay wet, such as during periods of frequent rain, the more likely fungal spores will land on the leaves, sprout and cause infections.
This is why gardening techniques that tend to discourage foliar fungal infections include:
1. Irrigating plants using devices that apply the water to the soil while leaving the foliage dry, such as soaker hoses or drip irrigation.
2. Planting plants that are prone to foliar diseases, such as roses, in locations where they receive morning sun that will dry the dew early.
3. Apply fungicides to plants highly susceptible to foliar fungal diseases, like vegetables, right after a rainy period ends to prevent fungal disease outbreaks.
Although the upper plant parts can deal with rainy periods well, the roots are where most problems occur. Excessively wet soil (especially combined with warm temperatures) can create stressful, and potentially destructive, conditions for the roots of bedding plants, perennials, vegetables, shrubs and even trees — especially newly planted ones.
After it rains, the soil is saturated — pore spaces in the soil, which usually hold air, are filled with water. Normally, gravity quickly pulls the excess water out of the upper soil, restoring the proper balance. We help this out by planting flowers, vegetables and shrubs in raised beds.
But when heavy rains occur over several consecutive days, the soil tends to stay wet over an extended period. Plant roots need oxygen to be healthy, and when the pore spaces are filled with water for a lengthy period, the roots suffer and do not work properly. A sick root system leads to a sick plant.
This can also occur in container plants if they are watered too frequently or the drainage holes get stopped up, preventing excess water from draining out of the soil.
Wet soil conditions also encourage water molds (such as Phytophthora, Pythium and Rhizoctonia) to attack the roots or crown of a plant and cause rot. Root or crown rot can cause wilting, scorched or brown leaves, leaf drop, dieback or even kill plants.
Once infection occurs, little can be done to help a plant. Plants with succulent stems, such as impatiens and begonias; those that like cooler temperatures, such as geraniums and dianthus; and those that prefer drier, well-drained soils, such as Indian hawthorns, rosemary and Texas sage (Leucophyllum frutescens) are particularly susceptible.
Gardeners can take steps to help alleviate the situation. For one thing, adjust your irrigation systems that are on automatic timers. All automatic irrigation systems should have rain sensors installed. You can also turn off the automatic timer when the weather is wet and turn the system on only when drier conditions occur.
You should always keep your beds well mulched to control weeds and help maintain soil moisture. When the soil of beds is staying saturated, however, the mulch should be pulled back from around plants or removed from beds entirely. This will allow evaporation to help the soil dry faster.
Plants affected by wet soils or root rot may look wilted, even though the soil is moist. Leaf edges or entire branches may turn brown, and a bush may yellow and drop leaves. Shrubs showing these symptoms may ultimately be lost, but prompt soil aeration in the root zone could help in some cases.
Using a garden fork, drive the tines straight down into the soil about 8 inches and pull straight out in numerous places around the shrubs. Do not dig with the fork, but make as many holes as seems practical. This technique provides air to the roots and encourages the soil to dry faster. A metal rod or wooden dowel could also be used to make the holes.
Fungal diseases that attack the foliage of many plants are also encouraged by rainy weather. Black spot disease on roses is more common after rainy weather, but resilient landscape roses, like Knock Out roses, Peggy Martin roses and Drift roses, will recover without treatment. Cercospora leaf spot on crape myrtles will likely show up in the coming weeks as a result of recent rain and cause leaf drop.
Lawn diseases, such as gray leaf spot and brown patch, thrive in wet weather and are bound to be more active if we continue to get abundant rain. Leaf spots and fruit rots are likely to be more prevalent in vegetable gardens as well.
Snails and slugs thrive and reproduce rapidly during rainy weather. These pesky critters chew holes in leaves and flowers and are particularly fond of soft-leaved plants such as impatiens, begonias and hostas.
Properly applied baits can be very helpful in control efforts. Choose products that contain iron phosphate, as it is safer and less toxic than other types. You can even place a bowl buried up to its rim in the ground and fill it half full of beer to attract and trap many snails and slugs.
And don’t be surprised to see mushrooms growing in your yard. Rain encourages them to grow. They are harmless to your lawn and landscape plants. But if you consider them a nuisance, collect and dispose of them when they appear.
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