Autumn Pruning of Perennials: Determining When to Neat Up and When to Let Plants Be
In the world of gardening, pruning is a crucial task that can significantly impact the health and bloom of plants. However, it's essential to know when to prune, especially for certain perennials and shrubs that bloom on old wood or growth from the previous season.
Lilacs, for instance, are popular for their fragrant blooms. They bloom on old wood, forming their flower buds during the previous growing season. Pruning in summer or fall risks cutting off these buds, resulting in no flowers the following spring. Forsythia, another favourite, behaves similarly, and pruning it in summer or fall will remove flower buds and reduce next year's blooms. The best time to prune forsythia is immediately after it finishes flowering in spring.
Azaleas, Rhododendrons, and many spring-blooming shrubs like Pieris, Magnolias, Dogwoods, and Lilacs also set their flower buds on old wood. Therefore, fall pruning is discouraged to avoid losing next year’s flowers. Azaleas should be pruned in late spring or early summer, not in fall, while Rhododendrons are best pruned right after blooming to allow new growth that will produce buds for the following year.
Anise hyssop is another perennial that should stop being pruned by mid-summer because late pruning can increase its vulnerability to cold damage in fall.
Pruning these plants in fall can eliminate next year’s flowers and weaken the plants by removing dormant flower buds or encouraging tender growth prone to frost damage.
When it comes to trees and shrubs, pruning should ideally be done from late fall to mid-winter when they enter dormancy. Pruning during dormancy reduces the risk of shock or disease. Damaged, crossing, or dead branches can be pruned from trees and shrubs during their dormancy period to keep them healthy and well-shaped.
Plants with interesting shapes, heights, or attractive seed heads add a unique architectural element to winter gardens. However, when it comes to disease-prone plants, pruning is a must. Any perennials showing signs of fungal diseases, powdery mildew, or other infections should be cut back and disposed of properly to prevent the disease from overwintering.
Leaving diseased plants untouched allows pathogens to linger in the garden, reinfecting plants next season.
Amy Grant, a professional chef and caterer who has been gardening for 30 years and writing for 15, specializing in culinary gardening, advises waiting until early spring to prune these plants, once birds have moved on and insects emerge. Tender herbs, like lavender, are best left unpruned until new growth appears in spring.
Many perennials, such as coneflowers, Joe-Pye weed, black-eyed Susans, ornamental grasses, and sedums, should not be pruned in the fall because they provide food and shelter for birds and wildlife during winter. Using clean, sanitized tools during pruning helps prevent the spread of disease.
Decayed leaves from a small patch of undisturbed garden can enrich the soil with valuable nutrients in spring. A small patch of fallen leaves, uncut stems, and organic mulch can create a winter habitat for beneficial insects like ladybugs and lacewings.
Leaving tall perennials and ornamental grasses untouched provides aesthetic appeal and habitat for overwintering insects. In colder climates, leaving flowering perennials to naturally fall over and decompose can provide a layer of insulation for roots during winter.
In summary, pruning at the right time is crucial for maintaining the health and bloom of perennials and shrubs. The table below provides a guide for the best pruning times for several key plants:
| Plant | Reason to Avoid Fall Pruning | Best Pruning Time | |--------------------|---------------------------------------------|-------------------------------| | Lilacs | Bloom on old wood; fall pruning removes buds | After flowering, late spring | | Forsythia | Bloom on old wood; fall pruning removes buds | Immediately after flowering | | Azaleas | Bloom on old wood; fall pruning removes buds | Late spring to early summer | | Rhododendrons | Bloom on old wood; fall pruning removes buds | Right after blooming | | Anise Hyssop | Late pruning increases cold vulnerability | Spring to mid-summer |
- When pruning the popular blooming shrub Lilacs, it's best to do so after their flowers have wilted in late spring, as they bloom on old wood and pruning in summer or fall could cut off their buds, resulting in no flowers the following spring.
- Similarly, when it comes to Forsythia, another favorite, it's essential to prune it immediately after it finishes flowering in spring to avoid removing its flower buds, which would reduce next year's blooms.
