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Garden Help Desk: Correct shrub care will increase forsythia blooms

By USU Extension - Special to the Daily Herald | Mar 23, 2024
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This forsythia hedge has very few flowers because the hedge is crowded with invasive trees and the remaining forsythia branches were sheared back in the fall, removing the spring-flowering wood. The hedge should be pruned after spring bloom and the invasive trees should be removed.
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Delay pruning spring-flowering shrubs like this fragrant lilac until after the blooms have faded so the shrub will have all summer to produce flowering wood for the following spring.
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Fall-flowering Beautyberry has inconspicuous flowers, but the showy fruits add striking color to the fall landscape. Pruning this shrub in the late spring or summer would eliminate much of the flowering wood. Late winter or very early spring is the time to prune fall-flowering shrubs like this.

Forsythias are blooming!

The forsythia shrubs in my neighborhood are blooming. That means it’s time to apply a preemergent herbicide to lawns that have problems with pesky annual and biennial weeds. If you live in a cooler part of Utah County, you should be seeing bright yellow forsythia blooms any day now.

And speaking of forsythias …

My forsythia hedge hasn’t bloomed very much for the past few years. It gets a few flowers, mostly on the inside of the shrub, but not many. How can I get it to bloom better?

There are a few possible reasons why you aren’t getting the masses of yellow blooms you used to see on your hedge. The most common reason with flowering shrubs is pruning at the wrong time. Flowering shrubs are usually classified as either spring-blooming or summer-blooming for pruning purposes, and it’s important to prune them at the proper time.

Spring-blooming shrubs produce their flowers on the twigs and branches that grew the summer before. If you cut back your forsythia shrub or shear back your hedge in the fall, you’ll be removing most of next year’s flowering wood. If you wait to prune until after the shrubs bloom, your shrubs will have all summer to produce flowering wood for the following spring.

Summer- and fall-blooming shrubs bloom on wood that grows from early spring until bloom time. If you prune during the spring or early summer, you’ll be pruning off some of your flowering twigs and branches. Prune these shrubs in late winter or early spring and they’ll have all spring and early summer to produce flowering wood.

You can completely prune out diseased or damaged branches at any time of the year (don’t leave stubs), but in general, you should avoid pruning in late summer or early fall when pruning can stimulate new growth that won’t be hardy enough to survive the winter.

Sometimes flowering shrubs don’t bloom because pruning has been neglected. It’s helpful to do at least some pruning every year. If a shrub is never pruned, it will eventually be crowded with branches that are too old to produce any flowering wood. Renewal pruning is a good way to promote flowering, encourage good air circulation and maintain the natural shape of the shrub. By completely pruning out one third of the tallest branches every year, you’ll always have young, productive growth on your shrub.

The third most common reason for poor hedge performance is invasion by other woody species. There are some trees and shrubs that reseed freely, and if any of these species are growing near a hedge (or any other landscape area), it’s quite likely they’ll show up in hedges, shrub beds and gardens. Sometimes these woody invaders aren’t noticed until they are well-established and crowding out the original shrubs.

I can see in your photos that your forsythia hedge has experienced both the pruning-at-the-wrong-time and the invasive-woody-plants problems.

Changing your pruning schedule will be easy. Simply wait until after the flowers dry and die on the shrubs and then shear back your hedge. You’ll get new growth all summer. Leave the new twigs and branches and you’ll be rewarded with bright yellow blooms next spring.

Dealing with invading species will be more difficult in a dense hedge like yours. In some areas of your hedge, it’s easy to see the difference between the shrubs and the young trees. Cut off as many of the trees as you can now while it’s easier to see what you’re doing. Carefully treat the cut surfaces of the stumps right away with glyphosate (KillzAll, Ultra-kill, Roundup, etc.) if you can apply the chemical without getting any on your forsythias. Once the forsythias leaf out in a few weeks, it will be harder to see and work around the base of the shrubs.

It may take a few years to get these invaders cleared out of your hedge, but stick with it. Inspect your hedge each spring and fall, especially along the soil line, watching for new invasive saplings, and remove them before they become established and deeply rooted.


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