Ask the Expert: Is it the right time to prune shrubs?

Editor’s note: Throughout the growing season, Mike Hogan, OSU Extension educator for Agriculture and Natural Resources in Franklin County, will answer gardening and home landscape questions submitted by Dispatch readers. Send your questions to hogan.1@osu.edu .

Q: I need to prune an arborvitae hedge because it is too tall. I’d like to remove 3 or 4 feet off the top of these shrubs. When is the best time to prune them?

A: Late winter or early spring is generally the best time to prune conifers (needled evergreens) such as arborvitae, but these plants do not respond well to heavy pruning at any time of the year. Removing such a large portion of these plants will result in a permanent bare area on the plant.

Once the foliage is removed, it will not regrow, even if sunlight now reaches the interior due to the pruning cuts. Unlike broadleaf evergreens such as boxwood or holly, needled evergreens do not have dormant buds along the older stems lying in wait to grow if the branch or foliage beyond them is removed. This is why deer damage on arborvitae is so visible, damaging and permanent.

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Reducing the height of a tall arborvitae will stunt the top growth and give it a permanent flat top and may stress the plant, making is susceptible to decline due to insects, disease pathogens or an overall reduction of vigor.

A better approach may be to remove the existing arborvitae shrubs and replace them with a smaller conifer. There are lots of compact and dwarf conifer varieties on the market these days. There are also other conifers such as yews, which are able to produce new growth after heavy pruning.

Q: We have three large blue-spruce trees in our yard and over the years, each of them has developed dead branches on different parts of the tree. One of these trees was completely dead, so we removed it. Is there a way to save the other two trees?

A: Colorado blue spruce ( Picea pungens ) is native to the mountains of Colorado. Over the past half-century this tree has become popular with homeowners, landscapers and even Christmas-tree growers in Ohio for its distinctive blue-green color and conical shape. Unfortunately, because they are native to the west, they are adapted to grow in drier climates with more wind and cooler temperatures, conditions not typical to much of Ohio.

Recently, we have seen a frequent decline in blue spruce in Ohio due to a variety of insects, disease pathogens and environmental conditions. While blue spruce is favored by insects such as bagworms, white-vine weevil and spruce spider mites, much of the damage we see on blue spruce in Ohio is due to a fungal pathogen called rhizosphaera. This fungal disease typically attacks the lower branches of the tree first and eventually spreads throughout the entire tree, killing it slowly over several years.

If identified early, before entire branches have died, trees infected with rhizosphaera will respond to a treatment with a fungicide containing chlorothalonil. Two applications of a fungicide for two or three consecutive years may be needed for complete control. The first application should be made just after bud-break in the spring, followed by a second application three to four weeks after the initial application.

Improving the environment around the trees may also help. Increasing airflow by proper spacing when planting, or removing the lower limbs of blue spruce can also help reduce incidence of the disease. Removing infected needles from beneath the tree can also reduce further infections.

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Q: Should a rain garden be watered when rainfall is sparse? Plants in our small rain garden are browning this summer.

A: Most plants require soil moisture in order to grow — for photosynthesis, for cooling and to transport minerals and nutrients from the soil into the plant. Plants in a rain garden are no different and will require supplemental watering during periods when rainfall is lacking.

We have a large rain garden on-campus which we have been watering recently because the herbaceous perennials such as cardinal flower, asters, iris and others were wilting and browning due to lack of soil moisture. Trees and shrubs planted in a rain garden can usually thrive during droughty conditions without supplemental watering, but herbaceous perennials with their smaller root systems will definitely benefit from supplemental irrigation.

This article originally appeared on The Columbus Dispatch: Ask the Expert: Is it the right time to prune shrubs?