Proper Timing Key When Pruning

Trees and shrub species vary in their prime time to be pruned. Some species have a wide window of opportunity for pruning, while others have just an abbreviated period for best results. Late spring is an optimum time for several species, with spring-blooming shrubs topping the list.

Forsythia and lilac are two of the more popular spring-flowering shrubs in landscapes throughout Wisconsin. Spring-blooming shrubs like these form their flower buds the previous year. To keep these shrubs blooming profusely each season, regular renewal pruning is essential. The objective is to remove older stems, which become decreasingly efficient for producing flowers, allowing new stems to be produced each season.

 Renewal pruning involves selecting and then cutting out older stems near the base, which then promotes growth and development of younger stems. Remaining stems may need some minor trimming, often referred to as “heading back,” to maintain a desirable overall shape. For spring-blooming shrubs, ideally do this within about two weeks after blooming. Therefore, do so ASAP on forsythia, as they have already completely bloomed out. As lilacs complete their blooming process, they will be ready for renewal pruning.

Renewal pruning is also suggested for red twig and yellow twig dogwood. Removing older stems with fading bark color promotes more vibrant colored new stems to develop. Early spring— while dogwood shrubs are still dormant—would be ideal timing.

Depending on where you are in Wisconsin, another optimum pruning window set to open is for pines. Elongation of the candles, or new growth found at the ends of the branches, is key to determining when to properly prune pines. Pinching back the new candles right after they have elongated results in creating denser, more compact pines, especially on younger plants. Unpruned pines tend to have larger spaces between branch whorls and look more open.

Many shade trees can still be pruned, but one species to absolutely avoid pruning now are oaks. The concern is oak wilt disease, caused by a fungus, which may be carried by sap beetles attracted to fresh wounds made by pruning. Wait until dormancy to prune oaks to avoid the threat of infecting oaks with oak wilt.

For other shade trees, major pruning is best done before July 4th. The reason for this is that pruning stimulates new growth, and significant pruning in late summer could promote a flush of new growth that may not harden off properly for winter. Ideally pruning while still dormant is better, as it is much easier to see the structure of the tree and cleaning up is much easier without leaves on branches.