I am concerned about the impact our recent subzero weather may have had on my garden. Do you think many plants will die from it?
A. There are many factors that influence a plant's ability to survive unusually cold weather, and the simple answer is that you will not know until spring arrives what survived and what did not.
One of the most important factors is the hardiness of the plants in your garden. If they were rated hardy to USDA Hardiness Zone 5a (-20 to -15 degrees), at least they were rated for the temperatures we experienced in much of Western Pennsylvania. Then you should also consider microclimates. Gardens in urban areas such as Pittsburgh did not get the dramatic lows we saw in outlying areas. And there are microclimates in outlying areas, created by buildings and pavement, bodies of water, hills and valleys, and other natural features that can help moderate or exacerbate such severe cold. Remember that cold air behaves much like water -- it tends to "run" downhill and collect in low-lying areas.
It helps that this winter has stayed seasonably cold pretty much since it started, rather than big temperature swings that have marked Pittsburgh winters in recent years.
Plants are better off when it gets cold and stays cold through the winter, and they attain their maximum cold hardiness in January and early February. While humans enjoy those warm spells in winter, they can cause plants to lose some of their cold hardiness, which makes them more susceptible to severe damage from exceptionally cold weather.
Soil drainage is another important factor. Well-drained soil that allows water to drain away from the roots and crowns of plants improves their winter survival greatly. Conversely, poorly drained soil that allows water to pool around the roots and crowns of plants is a recipe for disaster. Do not assume that sloped sites have good drainage. Pockets of deep clay, drainage from natural springs and runoff from building sites can cause poor drainage even on slopes. It can be frustrating because an area can differ in drainage patterns practically from foot to foot.
Cultural practices also have a bearing on a plant's winter hardiness. Plants that receive optimal moisture and fertility and protection from insect and/or disease problems during the growing season are healthier and will fare better through the winter. Plants suffering from drought stress, nutrient deficiencies or insect and/or disease problems may not survive the added stress of our extra-cold winter.
Proper management practices help plants harden off for the winter. Avoid pruning and fertilizing trees and shrubs in late summer and early fall. These practices stimulate succulent growth that may not harden off before cold weather arrives. Do not fertilize perennial herbaceous plants in late summer and early fall for the same reason.
It may seem counterintuitive, but good snow cover is a great insulator. Unfortunately, we had unseasonably warm weather just before the first polar vortex descended on us, so there was very little snow cover, at least in and near Pittsburgh. We did have more snow for the polar vortex's second visit, but the cold stayed with us much longer. The impact of lingering cold can range from dead flower buds on plants such as forsythia and bigleaf hydrangea to plants that die above the snow line but are fine where they were under snow.
Finally, when buying woody ornamentals, make sure that seed-grown plants originate in the same zone or a colder zone. Seed from plants grown in warmer climates may produce individuals that are unable to overwinter in colder climates, even though the species is considered hardy. Red maple (Acer rubrum) is a great example because it ranges from Quebec to Florida. Offspring of plants growing in Quebec are unlikely to tolerate the heat and humidity of Florida, and offspring of plants growing in Florida are unlikely to survive the winter (especially this winter!) in Quebec.
Send questions to Sandy Feather by e-mail at firstname.lastname@example.org or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.