Get container plants ready for approaching winter

Several days ago, in the evening sky, a hazy white circle with defined scarlet edges surrounded the creamy white glowing half moon. This phenomenon is the product of illumination through ice crystals in the cloud layer above and, some say, is a sure sign snow is soon to arrive.

In this case, the precipitation turned out to be more liquid, coupled with a springlike layer of dense ground fog. Yet all other environmental cues do point to the onset of winter.

While the duties involved in tucking the garden to bed for the winter may be fairly straightforward, the needs of potted plants in the patio garden or on the doorstep may not be so obvious.

Container plants have special needs during the growing season. Foremost, a suitable growing medium – one that offers a sound balance of draining excess moisture and holding an adequate amount of moisture and nutrients – goes a long way in promoting a healthy growing environment for a potted plant. When it comes to overwintering a container plant, however, what matters most?

First, it’s important to note that not all container plants are candidates for overwintering. Naturally, the life cycle of annual plants such as petunias, snapdragons, begonias and vegetables dictates they live only one year before they die. Annuals seeded or purchased at a greenhouse in spring may be overwintered indoors in the home, but they may be infected with disease or insect pests and conceivably can introduce those pests to treasured houseplants.

At the end of the growing season, after a killing frost, annual flowers and vegetables should be removed from their containers. Potting medium should be added to the compost pile, not immediately recycled as medium, since it may harbor disease or insect pests. Also, medium may lose some of its beneficial structural elements during the growing season and may not be as effective as it once was at draining or holding water.

Containers used in the patio or doorstep garden may harbor fungal pathogens or insect eggs or larvae. They should be cleaned at the end of the growing season to rid them of infectious elements before storing. Create a 10 percent chlorine bleach solution by mixing one part bleach into nine parts water. For example, measure 9 cups of water into a basin and add 1 cup of bleach. Using protective gloves, scrub clay and plastic pots with this solution.

Containers, particularly clay pots, should be stored indoors during the winter months. This protects them from the winter elements. While plastic pots aren’t particularly vulnerable to winter weather, clay pots are porous and, if subjected to moisture, will crack during periods of freezing and thawing.

Now, what about those potted plants such as miniature roses, azaleas, hydrangeas and rhododendron that were purchased or given to you as gifts over the spring and summer? Naturally, one would want to overwinter these plants, enjoying their beauty for years to come. A word of caution: Some woody ornamental potted plants and herbaceous perennials are not hardy to Maine zones. Carefully examine the tags or information provided with the plant. Be sure the plant tissue is hardy to your growing zone.

Some gardeners opt to overwinter marginally hardy plants indoors in a basement, cellar, garage or shed that is heated to above freezing all winter. It is possible, however, to overwinter hardy potted plants outdoors in Maine. The challenge is to protect roots from the fluctuating or intolerable cold temperatures aboveground.

Place plants in a trench in the ground on the protected (south or southeast) side of a home or outbuilding. Backfill around the container with soil and cover the very top of the pot with straw or compost mulch or soil. Avoid overwintering plants in clay pots that are likely to crack and split during the freezing process, opting for plastic pots or simply a bare root ball. Plants should be adequately irrigated but not saturated up until a hard freeze occurs and should then be topped off with a thick layer of straw or leaf litter mulch for extra protection over the winter months.

Keep in mind that mulching of any plant – in a container or in-ground – is a measure taken to protect plants from potential repeated thawing and freezing. Mulch plants after the ground has thoroughly frozen, “sealing in” the freeze until the warming months of spring.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, 512 North Ridge Road, Montville 04941 or e-mail dianagc@midcoast.com. Selected questions will be answered in future columns. Include name, address and telephone number.