By Kirsten Ann Conrad, Agriculture Natural Resource Extension Agent, Virginia Cooperative Extension
As the days grow cooler and shorter and flowers fade, there are still tasks for the home gardener. Whether you grow plants in containers on a patio, in window boxes, or a small, urban garden space, you’ll want to take protective measures for your herbaceous (perennials or annuals) or woody (evergreen or deciduous) plants.
Perennials are those that, under optimal conditions, will go dormant, survive the winter, and regrow in the springtime. Perennials grow flowers and produce seed/fruit every year and continue to do for many years. Woody plants that are acclimated to our climate typically go dormant in the winter while waiting for sustained warmth and accumulated degree days to reach a critical mass.
Overwintering perennials is a matter of making sure the roots have good drainage, continue to have soil moisture to draw on over the winter, and are somewhat protected from wind.
Plants that are not cold-hardy to your locality’s USDA hardiness zones need additional winter protection or should be moved into an unheated garage or other indoor space. Most of Arlington and Alexandria are in Zone 7a, but in enclosed spaces or next to water or areas protected by buildings that block the wind and create a heat island, you may be able to use plants that are hardy to Zone 7b.
Annuals are herbaceous plants that grow, flower, produce seed/fruit. and die over the winter. They are planted after danger of frost is past. Commonly called ‘bedding plants,” this large group of mostly non-native tropical species have many cultivars that provide great variety of color, form, and flower type.
Annuals are favored for pots and window boxes because they typically have a very long bloom time. One does not overwinter annual plants, but folks sometimes like to save the seeds of these plants for replanting the following year. Annual plants can be removed from the soil once they are killed back by cold. Some plants sold as annuals — one example is snapdragon — will occasionally survive the winter. Many other annual bedding plants including petunia, impatiens, and alyssum to name a few, and will successfully reseed if encouraged to do so by leaving bare ground under maturing seed pods.
Window boxes and or plant containers, if not planted, should ideally be emptied and placed indoors. Some masonry pots suffer from expansion and contraction during freeze and thaw cycles and will break if left outside. Replanting pots with fresh soil every few years is good for replenishing organic matter of the soil mix.
Bio-regionally native plants generally require no special treatment to survive the winter. Non-native trees like tropical figs or hibiscus are sometimes winterized by wrapping them in wind-proofing plastic or other materials to insulate them from the drastic temperature swings that we experience.
The No. 1 winter threat to evergreen woody plants is desiccation of leaves, stems, and trunk. Living tissues of the plant, especially the roots, continue to grow in the winter. Newly planted trees need 1 inch of water per week – even in winter — for two years. Remember: Long periods of drought can occur in winter as well as summer.
The key to survival is giving trees adequate moisture before winter freezes the top inches of soil around them. Once the ground is frozen, watering is more difficult. Up to 3 inches of mulch can be applied around the planting zone that will protect roots from freeze/thaw cycle and prevent water loss.
Smooth-barked deciduous trees in small gardens or containers that are in full sun are susceptible to sunscald damage called southwest injury. This happens when very cold nighttime temperatures quickly follow a sunny afternoon when the winter sun angles in from the south and warms up the trunk or a small, enclosed area with a heat sink like a masonry wall or concrete patio. This fast and extreme temperature change causes permanent injury as the bark splits. To avoid southwest injury, insulate lower limbs and tree trunks or move the tree into the shade before freezing temperatures arrive.
Many plants have very specific overwintering cultural requirements. For example roses: Cut back tea roses to short major stems and then mulch heavily to protect the graft junction. Knowing your plant and its cultural needs is essential to successful overwintering.
Where pest control has been a problem, heavy pruning can reduce the overwintering of, to name one, scale insects. Ornamental grasses are sometimes overwintered by cutting them back to stubble to prevent the dead leaves from being messy. Hardy perennials and bulbs should be mulched to limit temperature fluctuation until early spring when excess mulch should be removed from the crowns.
So, just because it’s late fall or winter doesn’t mean you get a complete break from your gardening. And, if you are into house plants, indoor vegetable production, or microgreens, you can stay even busier than you are in the summer with tasks like plant propagation, seed starting, and winter sowing.
For more ideas on preventative pest management, winter sowing, or winter plant protection, see our Between the Rows newsletter or contact us at the Extension Master Gardener Help Desk at firstname.lastname@example.org