Is It Normal for Hedges to Turn Brown After Trimming?

Timing

• Trimming while shrubs are actively growing increases the odds you’ll damage branches, which can lead to dieback or brown foliage. Generally it’s best to prune hedges during their dormant period, or the part of the year when they aren’t actively growing. Many types of shrubs can be trimmed in late summer, but evergreens should not be pruned until winter or early spring.

Severe Pruning

• Trimming too much from a shrub stresses the plant because it cannot regrow from old wood. The tools you use are also important. Dull pruning shears can tear the branches and shears that are too large might injure branches you don’t intend to trim. Use a sharp knife or small hand shears to trim shrubs to avoid browning and avoid trimming the branches back too far. Many evergreen and conifer species that are trimmed back to the old wood cannot regrow new buds, and this can result in brown patches.

Environmental Factors

• If you trim hedges during poor conditions such as when weather is hot, cold or excessively wet, or when winds are high, regrowth of the trimmed foliage might be inhibited and this can lead to brown spots. Never trim hedges during hot or dry weather when drought or sunscald could damage the foliage. Avoid trimming in the autumn when strong winds can dry the trimmed foliage. Brown patches may eventually fill in with new growth, but the regrowth process can take several years if the problem is severe.

Water Stress

• If a shrub’s water needs are not met, it becomes stressed. Evergreen shrubs naturally shed worn out leaves or needles, but under drought conditions, this process speeds up because the plant drops old foliage to conserve moisture. Trimming at this time can lead to brown spots as the plant is further stressed. Check the soil around the trunks of shrubs. If the soil is wet, do not water until the surface feels dry to the touch. If the soil is dry, thoroughly water the soil around the drip line to correct the problem. Water hedges deeply about once each month to prevent drought stress, and mulch around the shrubs, but not against the trunks, to retain soil moisture.

Pests

• Sucking pests such as aphids and scale insects feed on plant sap, which can cause recently trimmed hedges to develop brown spots. They also leave honeydew behind, which leads to the formation of black patches of sooty mold. Scale insects are often controlled by natural predators in the landscape and do not typically require chemical insecticide treatment. Remove aphids and honeydew with a strong spray of water, but spray early in the day so the shrubs have time to dry in the sun. Severe infestations of sucking pests may require foliar applications of insecticidal soaps or horticultural oils. Typically a 1- to 2-percent solution of oil in water is adequate. These insecticides are less likely than other chemicals to harm beneficial insects such as bees, but they typically kill only the insects present at the time of spraying, so you might have to repeat applications.