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.