For: Saturday, Dec.11, 2004
By: Daniel E. Mullins
Extension Horticulture Agent
Time to Protect Potted Perennials, Shrubs
and Trees
It’s time for gardeners to prepare for the season’s first hard freeze. Though most landscape plants survive our relatively mild winters on their own, container grown patio and deck plants require special consideration. These are typically special perennials or valuable tropical species and are worth the effort.
Extra cold protection is needed because plants grown in containers are more susceptible to serious damage from freezes than the same plants grown in the ground. This is because potted plants’ entire root ball is above ground and is fully exposed to outdoor temperatures. Added to this is the fact that the roots of many plants are more sensitive to freezing than the same plants’ leaves and stems.
There are only two options when protecting these valuable container grown
ornamentals – move them indoors or leave them outside, while providing as much
protection as possible. Obviously,
it is best to move them inside a greenhouse,
It is sometimes not possible or practical to move all potted plants indoors due to a lack of space. Also, some of these plants are big, older specimens and the sheer size and weight makes handling difficult.
All is not lost if the choice is to leave part of the potted plant collection outside. However, there are some things that can be done to help ensure that they survive the winter in good condition.
First, move plants to a location that is protected on the north side by a house, outbuilding or solid fence. Choose a spot that does not have an eastern exposure. Such an exposure allows direct sunlight to strike plants early, while temperatures are below freezing. This condition often causes more damage than the freeze itself.
Once the plants are grouped, place the pots as close together as possible. This is known as “jamming” and it helps to hold warmth during an extremely cold night.
Use a coarse material such as pine needles, shavings, bark or pine needles to help insulate the pots, holding ground heat. Place the material against the containers from the ground to several inches above the top of each. Also, place the material in the spaces between containers. The principle is to make the group of potted plants into a single, large insulated unit.
Water as needed to keep the potting mixture moist during the winter. Well watered plants are much more resistant to cold injury than those suffering from drought stress.
The covering of plants can be helpful in protecting against cold injury, but this practice should be limited to extremely valuable cold sensitive plants that can’t be brought indoors. Covering requires that an opaque material be used and it must be placed a few hours before extreme cold is expected and removed as soon as the temperature rises above freezing.
The covering material must completely envelope each plant and drape to the ground, where it is sealed with soil or heavy objects. The benefit of proper covering is that it traps ground heat, keeping the temperature under the covering several degrees warmer than outside. Just draping a sheet or blanket over the canopy of a plant without allowing it to extend to the ground on the other hand, does no good at all.
Covering plants can be time consuming and requires a commitment. Done properly, it will require the timely application and removal as many as a dozen times during the winter. I trust that I have sufficiently discouraged most gardeners from covering plants. There are many other more rewarding jobs for gardeners.