By Hilary Hart
Winter dormancy is the best period for pruning many trees and shrubs and for cutting back most herbaceous perennials. However, full dormancy is late this year, and with the warm weather we’ve seen some rather strange phenomena: fall color and spring blooms on the same shrub, pineapple sage providing Christmas color, and star magnolias blossoming on New Year’s Day. Winter, nevertheless, is finally here, and the recent cold snaps will push plants into full dormancy. If you’d like hands-on pruning instruction, consider signing up for Garden*Hood’s Winter Pruning Workshop held Saturday, February 6. In the meantime, here are some tips for planning your winter pruning strategy.
Plants to Prune in Winter
In general, the focus of winter pruning is on deciduous trees, shrubs and perennials. Evergreens, on the other hand, should in most situations be pruned in the growing season. Because they never become fully dormant, evergreens may suffer tip burn if pruned in the winter.
How Do You Know If a Plant Needs Pruning?
If during the growing season a plant is looking spindly, scraggly, depleted, or overly dense, pruning is in order. Some plants in particular are subject to the predations of insects and to fungal diseases if they have inadequate airflow.
In addition to cutting for health, plants may be pruned for shape and appearance. The cuts you make now direct the plant’s growth come spring, so think about the shape you want to create. For instance, if you want a saucer magnolia (Magnolia × soulangeana) or sweetbay magnolia (Magnolia virginiana) that is dense and shrub-like, retain several trunks and cut the branches to stimulate bushy growth. If you’d prefer a more tree-like from, select a single trunk and cut away the rest.
Hydrangeas While most hydrangeas are deciduous, they are not all good candidates for winter pruning. Depending on the species, deep cutting in fall or winter can result in fewer blooms and undermine the plant’s cold hardiness.
Hydrangea paniculata (panicle hydrangea) blooms on the growth produced during the upcoming growing season (known as “new wood”) so you can prune in late winter to early spring.
Hydrangea arborescens (smooth hydrangea) can be pruned to the ground line in winter or early spring, as it blooms abundantly on new growth.
Hydrangea macrophylla (mophead and lacecap hydrangea) buds form on “old wood” in fall and winter, so pruning in late summer, after the flowers have faded, is best. Don’t wait until late fall (September) to prune deeply because the new growth will not have time to mature and is then susceptible to winter dieback. The same goes for H. serrata. H. macrophylla ‘Endless Summer’ is a repeat bloomer that flowers on both old and new wood. However, avoid a late fall or winter pruning to preserve its hardiness.
Hydrangea quercifolia (oakleaf hydrangea) should also be pruned in late summer, after the blooms are spent.
Grasses Wait to cut back ornamental grasses until the first signs of new growth in March. Grasses are subject to crown rot, especially during wet winters, so it’s best to leave them alone until they are actively growing again. Prune to a height of about 3 to 6 inches in early spring.
Roses Prune bush roses (e.g., Knock Outs, Rosa mutabilis and others with a shrubby habit) between Feb 15 and March 15. If your rose has been pruned every year, prune back the size of the plant by 1/3. A completely overgrown rosebush should be reduced by 1/2 to 2/3. Kacey will provide more detailed rose pruning instructions during the Winter Pruning Workshop on Feb 6.
There is some debate about what makes certain square-stemmed plants less able to endure winter. Some are of the opinion that square stems allow water to enter, freeze and kill the plant. To be on the safe side, avoid late fall and winter pruning of lantana, salvia, butterfly bushes and other square-stemmed plants.
Most of the perennial herbs we enjoy are in the Lamiaceae family, a common feature of which is a square stem. So leave your sage, mint, rosemary, lavender, thyme, and oregano alone for the winter. (Cutting stems for culinary use is just fine during winter.)
Giant Leopard Plant (Farfugium japonicum ‘Giganteum’) Cutting back Farfugium in winter can result in the plant’s death. Yes, Farfugium looks absolutely miserable during dormancy, its huge leaves brown and crumpled on collapsed stalks. Nevertheless, resist the urge to cut away the dead foliage! Instead, consider adding evergreen companion plants such as hellebores, creeping plum yew, autumn ferns, butcher’s broom, or Rohdea to enliven the the scene around your dormant Farfugium.
Take Stock Pour yourself a hot cuppa, put on your hat and coat and tour your garden. You can really see the architecture of a tree or shrub once it’s bare of leaves, and this makes it easier to identify what needs thinning and shaping.
Ready Your Tools Take time to assess your arsenal this winter, and if you find yourself in need of new tools, consider donating your old ones to Oakland Cemetery. (No, not to be buried! Oakland hosts hundreds of volunteers in its gardens each year, and it is always in need of more tools. Donated tools will be cleaned and repaired and put to very good use.)
Only use tools that are clean and sharp. Using dull, dirty tools can spread disease around your garden. Clean cuts are essential to plant health, plus the job will go faster with sharp edges. Once tools are sharpened and lubricated, wipe the blades with a rag dampened with rubbing alcohol to kill disease-carrying microbes.
Also remember to sterilize your pruners between each plant as you undertake pruning tasks in your garden. Pruning in winter when plants are dormant greatly reduces the risk of spreading disease, but it’s still a good idea to get in the habit of wiping down your pruners between plants.
Always use the right tool for the job. Coercing a tool into doing a task for which it wasn’t made can damage both the tool and the plant. If, for instance, you find yourself straining to cut a branch with a pair of hand pruners, move up to loppers or even a pruning saw.
Once you start cutting, work from the inside out removing deadwood, suckers, and branches that cross and wound one another, as well as the small branches and twigs that comprise the dense mass of the tree or shrub. Open the canopy and accentuate the plant’s natural growth habit. Be prepared to take your time. Cut some, step back, assess. Repeat.
Pruning can seem daunting, but once you understand the science behind it and learn which plants take to which types of pruning, it can be an incredibly rewarding way to rejuvenate your garden. Finally, be prepared to accept the “happy accident.” Some of the most endearing garden elements you’ll ever encounter may be the result of pruning mistakes.