December Monthly Gardening Guide for Pima County

Guide
Authors
Paul Larson
Laurie Vance
Deborah North
Publication Date: January 2025 | View PDF

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.

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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 using tips from the article Frost Protection. 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. More information is available in Problems and Pests of Agave, Aloe, Cactus and Yucca.
  • 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. Read Protecting a Citrus Tree from Cold to learn more.
  • 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 following steps in Pruning Deciduous Shade Trees and Training and Pruning Newly Planted Deciduous Fruit Trees. 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. To learn more, refer to Harvesting Rainwater for Landscape Use and Landscape Management Practices to Optimize Passive Rainwater Harvesting and Plant Health.

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. Read the Flower Planting Guide for the Low Desert to learn more.
  • Seeds of cool-weather vegetables can be planted directly into the soil. Examples are leaf lettuces, peas, radishes, carrots, chard, kale, parsnips, and beets. Learn more about planting in Ten Steps to a Successful Vegetable Garden.

Fertilizing

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.