Guide
December's moderate daytime temperatures make gardening more comfortable and enjoyable, but don't be fooled—freezing
temperatures are still possible for up to three more months. Be prepared to protect sensitive plants from cold nights by moving
them indoors if possible or using frost cloth and other cold-protection methods. Avoid planting or transplanting cold-sensitive
plants and be sure to know which plants can tolerate pruning or fertilizing during the winter months and which cannot.
Maintenance
- If you haven't already, move cold-sensitive potted plants indoors. This includes adeniums (desert roses) and other
vulnerable succulents like pachypodiums and certain euphorbia. Place them in a sunny spot but keep them away from
direct heat sources like vents. - Watch for cold nights and protect plants as needed. Many plants are sensitive to 40 degrees, while others can tolerate much lower temperatures. Knowing your USDA Plant Hardiness zone helps localize your temperature expectations for plant protection.
- Do NOT prune frost-damaged plants until the danger of freezing is past. Pruning now will only stimulate new plant growth,
which is not recommended. - Cap cold-sensitive cacti, such as totem pole, golden torch, Mexican fencepost, and young saguaro. Use a paper grocery
bag or large Styrofoam cup to protect the delicate growing tip from the cold. Do not use plastic in direct contact with the
plant; it will conduct cold not insulate. After the freeze, remove the cap, but keep it – you can reuse. - Pick exposed lemons and limes before a hard freeze. Some damage to these fruits on upper and outer portions of the tree
will occur when temperatures drop to 28 degrees. Other types of citrus fruits can handle cold temperatures better. - Begin harvesting grapefruit at the end of the month, picking only those you need. The longer they stay on the tree, the
sweeter they become. Fruit may be left on the tree for several months. Nearly all varieties of grapefruit that are suitable for
our location are frost-hardy. - Prune non-native deciduous trees and shrubs while dormant. Natives are best pruned when temps are warm.
- Delay pruning your spring-flowering shrubs until after they bloom.
- If you've previously had pest infestations on your roses, grapes, or deciduous flowering and fruiting trees, consider
spraying the foliage with horticultural oil to kill overwintering insects. For the best results, apply the oil after winter
pruning. Ensure thorough coverage, as the oil works by smothering insects hidden in the nooks and crannies of the woody
growth. - Keep poinsettias healthy by locating them away from drafts and direct sunlight. They prefer bright, indirect light during the
day and a cool location at night. Keep the soil evenly moist, as over- or under-watering will cause the leaves to yellow and
drop off. Fertilize monthly with a water-soluble “balanced” houseplant fertilizer. After all danger of frost is over,
poinsettias can be planted outside in a shady location. - Apply Btk (Bacillus thuringiensis kurstaki) if caterpillars are damaging your cabbage, lettuce, broccoli, or cauliflower. Btk is
safe for animals and humans but is lethal to all larvae of the Lepidoptera family (butterflies and moths). While effective,
use it with care to avoid harming beneficial species. - Now is a good time to consider planning or installing active or passive rainwater harvesting.
Planting
- Plant cold-hardy flowers into the ground such as snapdragons, nemesia, petunias, pansies, stock, alyssum, million bells
(Calibrachoa), and geraniums. Choose a sunny spot as the flower yields will be improved. - Seeds of cool-weather vegetables can be planted directly into the soil. Examples are leaf lettuces, peas, radishes,
carrots, chard, kale, parsnips, and beets.
Fertilizing
- Continue with mild fertilizing of winter vegetables.
- Over-seeded Bermudagrass lawns need applications of a nitrogen fertilizer once per month.
- Other uses of fertilizer are not advised. Fertilizers stimulate new growth that will be frost-sensitive.
Watering
- Adjust your outdoor irrigation frequencies (not duration) for December. If there are ample rains, consider turning your
system off. Keep an eye on your tender annual plants to ensure they are receiving adequate moisture. - If you brought potted succulents indoors to protect them, water these very sparingly during this time.
More Information
Extension Publications
For more information on any of these topics, search our database of Extension publications going back more than 100 years housed in the University of Arizona Campus Repository.