Q. It has gotten down to
30 degrees overnight here in Boulder City, NV. I'm worried about my little
6-foot lemon tree (in a 5-gallon pot on the porch). So I placed a queen-sized
bed sheet over the tree, and hung a 100 watt incandescent lamp from a branch
last night. I'm hoping that small amount of heat will raise the temps above
freezing. Is this necessary?
A. There are some
different reasons and different temperatures that will affect citrus in
different ways. Remember that low or freezing temperatures have two components;
the low temperature you reported of 30F and its duration or how long that
freezing temperature lasts. The longer a damaging
temperature lasts the more damage it will do. Meyers Lemon is more tolerant of
freezing temperatures than other lemons.
Hanging a light bulb that
gives off heat will help a tree if there is no wind or the wind is blocked. If
there is wind present then heat from the bulb helping is questionable.
Covering a tree with a sheet or burlap helps keep the wind off of the tree and
may trap some warm air under it for a short time. If the cold lasts a long time,
then eventually the air under the sheet will get to the same temperature. Again,
it’s a matter of duration. The more insulating properties this cover has, the
better it will be at keeping that temperature from dropping.
Open flowers, young fruit
and new growth are the most sensitive parts of any fruit tree. Open flowers of
any fruit tree, whether the fruit tree itself can withstand -20F or 32F will
always be damaged if not outright killed when temperatures hit 32F for very
short periods of time. If your tree had flowers that were open AND the
temperature around those flowers hit 31F for 30 minutes, the flower and any fruit
it could have produced are dead and gone.
Unopened flowers and young fruit are
slightly more tolerant of freezing temperatures but not much. If the
temperature dropped to 28F for 30 minutes, very young fruit will be killed and
these will drop from the tree. Larger fruit require lower temperatures but
still can be damaged depending on the “antifreeze” (sugar content) of the
fruit.
New growth will be damaged just below freezing as well. This is why I
tell people NOT to fertilize winter tender plants during the second half of the
summer.
Established trees are a
bit different. Each type of fruit tree has its own special tolerance to
freezing weather. All citrus is generally tender to winter freezing
temperatures. They are considered "semi-tropical". This is why growing citrus in the Las Vegas Valley is generally
considered risky. Young citrus are damaged more extensively at the same freezing
temperatures than mature citrus. Mature citrus has more “mass” and so it can
withstand the same freezing temperatures that might kill a young tree of the
same kind. Instead, larger trees will get some damage but they will grow back
from that damage.
So to answer your
question, yes, the light bulb helps. Keep the bulb from burning leaves and stems. But
ultimately it will depend on how windy it is, the duration of the freeze and
how insulated the cover is. It is best if the cover reaches the ground so that
as much warm surfaces can radiate heat back to the tree as possible.
