QI have five white pine seedlings, each about a foot tall, in 5-gallon cedar planters. I also have a shrub rose in a container. How can I overwinter these plants?
APlants in containers must have special protection to survive the winter. Even if the top part of the plant is hardy (which white pine certainly is, and the shrub rose may be), winter temperatures will kill the root systems. Plant roots are much less hardy than the top of the plant, and roots can be killed when temperatures drop to 25 degrees.
Plants in wooden or plastic containers can be overwintered by digging a hole in the garden and setting the entire container in the hole so the top edge of the pot is at or just below ground level. Fill soil in all around the pot to insulate the root system.
When the ground starts to freeze, put a thick layer of mulch (dry straw or bags of dry leaves) over the buried container. You cannot use this method for clay pots, because freezing temperatures can make them crack.
When temperatures rise sufficiently in the spring, dig up the pots and scrub them.
Setting pots into the garden is laborious, and you might be tempted to bring the pots indoors. But these woody plants need a period of cold dormancy to grow normally, so you can't bring them in the house as you could a potted geranium.
If you're lucky enough to have an old-fashioned root cellar where the humidity is high and the temperature hovers slightly above freezing (32 to 40 degrees is ideal), you have the perfect storage spot for the container plants. Unfortunately, almost no one has these storage conditions, and the basement of a typical modern house is too warm (over 50 degrees) and dry to store plants.
I've had reasonably good luck storing plants in containers in my unheated, attached garage. In late fall, I water each plant well and allow it to drain, then I place each container in a large trash bag and fill in around and on top of the container with loose, dry straw or leaves.
If the plants are small enough you may be able to get the whole plant into a bag; if not, tie the bag closed around the trunk of the plant. I push my bagged containers against the house wall, which is the warmest spot in the garage. This method of overwintering is not guaranteed, however, because your garage may be much colder than mine.
QMy Norway maple has a long crack down the trunk, which opens up during the winter, then grows back together in the summer. Is the tree in danger when the crack opens?
AYour maple has a frost crack. These cracks often occur when the tree bark experiences fluctuating temperatures in the winter. On a sunny day, the temperature of the bark on the south or west side of the tree can easily be 20 degrees higher than the air temperature. The temperature difference between the frozen north and heated south sides of the tree can cause the bark to crack vertically on the warm side.
Also, when the bark is heated and the temperature suddenly drops, the rapid cooling of the bark can damage tissue. This usually is called sunscald. The damage is often more of a sunken, cankered area on the trunk than a long crack. Both types of winter damage frequently are seen on smooth-barked trees such as Norway and red maples.
Trees that already have some internal decay may be more susceptible to frost cracks, so you may want to evaluate the health of your Norway maple. Once a frost crack has developed, it will tend to reopen every winter.
You may notice ridges of callus along the edge of the crack. Those are formed when the tree attempts to close up the crack by developing this callus during the growing season.
Frost cracks are not a major threat to the tree. A wound on a tree, however, often becomes an entrance for secondary pathogens. Severe cracking or sunscald also can affect the structural strength of the tree over the years.
-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.