Shrubs and evergreen trees can survive the winter in containers

During the last few months several gardeners have expressed an interest in planting bulbs, small trees or shrubs in an outdoor planter box or pot. Many want to create a focal point around their front door, or they want to soften harsh architectural lines with plants but find wood, concrete or asphalt where planting beds should be. Their questions is "Will shrubs and bulbs planted in containers survive the winter in our climate?"

In order to answer the question, you must keep in mind three plant characteristics. First, plant roots are not as hardy as plant tops. A particular plant may do very well planted in the ground in the region. But remember, the temperature around roots growing in a pot sitting on a deck, patio or sidewalk will be much colder than the temperature around roots growing in a garden or landscape bed. Therefore, the same plant growing in a pot may not be able to withstand our winters because of the soil temperatures in an above-ground container.

You can protect plant roots growing in containers from low winter temperatures in several ways. First, you can use a large container which requires a large volume of soil. The soil itself will act as an insulator for the roots in the container. The more soil, the better the insulating qualities.

You also can use thermolite pots that are now on the market. These terra cotta look-a-likes are made out of foam. This thick foam layer will act as an insulator for plant roots and protect them from plunging temperatures during the winter.

Another way to protect plant roots is by actually lining a planter with foam insulation. This works very well in square wooden planter boxes. Again this insulation will protect plant roots from cold winter temperatures.

Not only do plant roots need to be protected from cold winter temperatures, you must also realize that plants need water during the winter. This is especially true for evergreen shrubs that are often planted in containers. Evergreens transpire all winter long. If you do not keep adequate amounts of moisture in the containers' soil, these shrubs will suffer from winter burn and quite often die.

Finally keep in mind that plants growing in pots need an occasional application of fertilizer to keep them growing and thriving. Plants can often deplete nutrients in a pot. Since the roots are confined, they cannot grow into adjacent soils and find more nutrients. You must provide for them. I would suggest using a fertilizer such as 23-18-16 every two to four weeks during the growing season, and then only once every other month during the fall, winter and early spring.

I know of two businesses that have been successful with outdoor evergreen shrubs. They have kept in mind the need for insulation around roots, moisture and nutrients.

Send your gardening and landscape questions to Paul Schnare at P.O. Box 699; Cape Girardeau, Mo. 63702-0699 or by e-mail to news@semissourian.com.