Think Twice Before Pruning After A Rain. Here's Why

Pruning is an important way to keep your plants healthy and looking beautiful. But pruning after a rainstorm can have the opposite effect. Whether you're pruning shrubs or trees, in the spring or fall, summer or winter, it's best to wait 48 hours or more before pruning. Pruning exposes plants' inner tissues to the elements, leaving them vulnerable to pests and infections. Plants such as evergreen trees heal those wounds by producing resins which harden to create a scab over their wound. As with human wounds, it takes time for those resins to harden through evaporation. Moisture in the air can slow down that evaporation process, leaving plants vulnerable. So unless you need to remove dead, dying, diseased, or damage branches, save pruning for a sunny day.

Meanwhile, snails and slugs like moist environments and will more likely find their way to your plant's open wounds after a rainstorm. Rain, pooled water, and moisture in the air from evaporation encourage moss to grow on trees which fosters bacteria, molds, mildews, and other fungal diseases that enter the air, possibly landing on your plant's exposed surfaces. That said, you should still avoid the other extreme of pruning during a drought, which stimulates plants to expend energy healing their wounds at a time when they are already stressed from a lack of water.

If the worst time to prune is after a rainstorm, you'll have less opportunity to prune at different times of the year, depending on where you live. Generally speaking, precipitation averages are highest in the summer in most of the eastern half of the United States, the West Coast receives most of its rain in the winter, while precipitation rates are generally consistent throughout the year in the Mountain and Prairie States. But no matter where you live in North America, fall's shorter days and cooler weather can give unwanted organisms greater opportunity to take advantage of your plant's open wounds. 

As plants wind down their growing season in the fall, they produce less of the resin they use to heal themselves, which often contains antibiotics that fight off disease. In the summer, cloudy weather or high humidity mean rain takes longer to evaporate, while in winter, rain-soaked branches are also vulnerable to frost damage, making matters worse.

In short, the best time of year to prune your trees and shrubs depends on the type of plant. Fall is the best to prune shrubs that produce flowers on new wood, which is wood that grows in the spring. By contrast, shrubs that flower on last year's old wood should be pruned right after they've bloomed in late spring or summer. Deciduous trees should be pruned once they've dropped their leaves and gone dormant, so prune during winter into very early spring. It's best to prune evergreen trees and shrubs in early spring as the plants come out of dormancy and prepare to produce new growth. But whatever season is best for your plants, use sharp, clean shears in tip-top shape, and pick a sunny day.