December in the desert is the coldest month, but you can still plant some cold-hardy plants and winter annuals. It’s also time to get ready for the first freeze. Here’s a look at what you can plant this month, which plants you should stop watering, how to grow poinsettias, and more.
December is the beginning of true winter in the desert. (Fall went fast, didn’t it?) Every December, I have a hard time wrapping my head around these 2 facts:
- December is the coldest month of the year (although just barely).
- By mid-month (December 10th) the evenings start getting longer!
December Weather Outlook
The average highs and lows in Tucson and Phoenix are around 10 degrees cooler than November’s. Additionally, the sun continues to get less intense — the UV index in December is the second lowest of the year.
The chance of precipitation increases slightly, while the relative humidity increases to what would be considered “normal” levels.
Here’s a look at December weather averages in both Tucson and Phoenix.
December Weather Averages (Tucson)
| High temperature | 66°F |
| Low temperature | 40°F |
| Precipitation | 1.02″ |
| Relative humidity | 47% |
| UV Index | 2.8 |
| Wind | 12 mph |
December Weather Averages (Phoenix)
| High temperature | 67°F |
| Low temperature | 45°F |
| Precipitation | .74″ |
| Relative humidity | 43% |
| UV Index | 2.5 |
| Wind | 6 mph |
If you live elsewhere, enter your city or town in the search bar at TimeandDate.com to find your weather averages. And you can find your average UV index at UVIndex.io.
December Gardening Tasks
Here’s a snapshot of the tasks you can expect to do (or not do) this month.
| ☑️ | Be prepared for frost |
| ☑️ | Cut back on watering |
| ☑️ | Plant cold-weather plants |
| ☑️ | Get your holiday plants |
| ✖️ | Don’t fertilize |
| ✖️ | Don’t prune |
Read on for all the details and exceptions!
Be Prepared for Frost
There are a few things you can do to get prepared for frost.
Know Your First Frost Date
Knowing your first frost date is somewhat useful since it helps you know when to start being prepared.
But you might be disappointed to learn that there is no one “ultimate first frost date,” instead there’s a range of dates when it’s likely to happen.
According to PlantMaps.com, which gathers its info from the NOAA, the first frost date is likely to occur between December 1 and 10. This is based on the last 30 years of data. According to Garden.org, which discusses weather in terms of probabilities, there’s a 50% chance that Tucson will experience a frost by mid-December.
The earliest freeze on record in Tucson was on October 16th!
Not in Tucson? Find information on the first frost date in your area at any of the sources listed above.
Figure Out What Plants Need Protecting
Don’t assume that all your plants need protecting. Most desert landscape plants don’t need protecting, especially if they are native. It’s rare that a mature plant needs protecting — if it’s made it this far, it should continue to be OK!
You can find out which plants are frost-sensitve by referring to your favorite gardening book or online resource. (I hope our Desert Plant Library is one you have bookmarked.)
Know Your Precise Temperature Forecast
Second, follow the weather as close to home as possible. Typical weather forecasts will tell you what to expect at the airport — but I doubt you garden at the airport! I use Weather Underground which offers over 60 weather stations in Tucson to pick from.
Type your city or town in the search box, then click on “Change” and you’ll be presented with a drop-down list of “Nearby Weather Stations.” Review the map and choose the weather station nearest you.
Additionally, put a thermometer or two in various spots in your yard. You may be surprised to find how consistently cooler or warmer some locations are. Knowing this can make the difference between having to cover your plants or not.
Not sure of your “Zone” or why it matters?
The Importance (& Limitations) of Knowing Your USDA Hardiness Zone
Be Ready to Cover
Third, have what you need to protect your plants before the first freeze.
Frustratingly, cold fronts are often accompanied by wind and sometimes rain. So do a practice run to figure out how you’ll secure your coverings ahead of time. You don’t want to find your frost cloth in your neighbor’s yard the morning after a frost. (Yes, I’ve had that happen.) 🫣
You don’t have to use frost cloth. Many gardeners improvise with sheets or burlap, but frost cloth has some advantages. It lets in air and light, can be left up for days, and if there’s rain, it dries fast and doesn’t weigh your plants down. Towels and blankets can be too heavy for some plants.
Covering Cactus
It’s not unusual to see columnar cactus tips covered with red plastic cups or small Santa Claus hats for the holidays. Some people do this for decoration while others are trying to protect the tender tips from frost. Not all cactus need protection but totem pole, fence post, and young saguaros are among those that benefit from frost protection.
Cloth hats, paper bags, or styrofoam cups are good choices, but don’t use plastic cups! Plastic does not insulate — it conducts cold — and can cause more damage than leaving tips bare.
What To Do in the Garden in December
A lot of tasks — planting, pruning, watering, and fertilizing — are winding down this month.
What to Plant in December
You can still add plants to your garden this month that aren’t frost-sensitive:
- cold-weather annuals
- cold-hardy perennials, shrubs, and trees
- bare root roses and deciduous fruit trees
- cold-weather herbs and vegetables
You no longer have to worry that it will be too warm for winter annuals (which prefer a night-time low of mid-50s), so you can plant some of your favorite flowers including pansies, snapdragons, alyssum, lobelia, stock, dianthus, geraniums, and petunias.
It’s no longer advised to plant palms, perennials, cactus and other succulents, or any plant that is cold-sensitive. Cold soil, short days, and occasional freezes makes it hard for these heat-loving plants to establish. If you simply must plant these now, cover them when there’s a possibility of frost.
Watering New Plants
If you planted any trees or shrubs in October or November, continue to water them according to this University of Arizona recommended schedule for 8 weeks total.
| Weeks 1 & 2 | Every 3 – 4 days |
| Weeks 3 & 4 | Every 5 – 7 days |
| Weeks 5 & 6 | Every 7 – 10 days |
| Weeks 7 & 8 | Every 10 – 14 days |
Cut Back on Watering Established Plants
By now, you should have drastically cut back on watering frequency for all your established plants.
By the end of December, you can stop watering succulents such as cactus, agaves, aloe, hesperaloe, and desert spoon. Winter rains normally provide adequate water between fall and spring.
Never water succulents — either in pots or in the ground — just before a freeze. Too much water makes succulents more susceptible to winter freeze injury and can cause root rot. They don’t like cold, wet roots.
Not sure how to adjust your watering schedule for December?
These articles can help:
How to Water Desert Trees: How Often? How Much?
How to Water Desert Shrubs: How Often? How Much?
Watering Before a Freeze
Most plants benefit from watering before a freeze. The exception to this rule is cactus and other succulents. Their cells are already filled with water and too much causes cells to burst.
What to Fertilize
Don’t fertilize any landscape plants in December. Feeding them now can encourage new frost-sensitive growth. However, you can still fertilize flowering annuals once a month.
What to Prune
Basically, you can put your pruning tools away for a while since pruning your plants encourages new frost-tender growth. However, you can always prune any dead or broken branches on shrubs or trees, especially if they pose a hazard.
Additionally, you can continue to deadhead annuals to encourage flowering.
Check Your Citrus!
Depending on your microclimate and the variety you’re growing, some citrus may be ripe this month. Grapefruits are often ripe by the end of this month.
Color is not the best way to determine if citrus is ripe. When ripe, fruits should have a pleasant citrusy smell and have some heft when you hold them.
If you suspect your fruits are ripe, pick one and give it a try. It’s the only way you’ll know for sure. And remember that citrus, once picked, does not continue to ripen.
And don’t be in a hurry to pick oranges and grapefruit since “The longer on the tree, the sweeter it will be.” 😉
Time to Get Your Holiday Poinsettias
No discussion of December gardening would be complete without mentioning the iconic holiday plant poinsettia.
Poinsettia (Euphorbia pulcherrima) is a native of Mexico. It’s in this same genus as many popular succulent landscape plants including gopher plant, candelilla, lady slippers, Moroccan mound, and firesticks. Like all Euphorbias, poinsettias exude milky sap.
While you can pick up poinsettias just about anywhere, if you live in Tucson, treat yourself to a visit at Green Things Nursery. As the largest grower of poinsettias in Arizona, it is the place to buy poinsettias.
Growing Poinsettias
Most people treat poinsettias like cut flowers — enjoy their temporary beauty and toss after the holidays.
But if you like the challenge of trying to keep one growing all year and getting it to rebloom next winter, Green Things offers very specific online instructions on how to achieve this. They sometimes offer a free class on growing poinsettias before the holidays.
Can you grow poinsettias in the ground?
I’ve heard of gardeners growing poinsettias in the ground, but it depends on your microclimate. They don’t like extreme heat or winter freezes.
Poinsettia Fun Facts
Here are some about interesting facts about poinsettias:
- Every holiday season, over 70 million poinsettias are sold within a 6 week period.
- Poinsettias are America’s #1 selling potted plant, contributing $250 million to our nation’s economy.
- Over 100 varieties have been bred. They now come in a rainbow of colors –white, cream, pink, purple, orange, and yellow. There are also marble and bicolor varieties.
- Poinsettias are named after Dr. Joel Poinsett. He was the US’s first minister to Mexico who introduced the plant to the US.
- The red “flowers” are actually modified leaves called bracts.
- They can grow up to 15 feet tall. (My next door neighbor in Florida had one taller than her house!)
- Poinsettia can be toxic to pets, but is considered irritating rather than toxic to humans.
Other Holiday Plants
Christmas cactus, cyclamen, kalanchoe, and amaryllis are other colorful plants associated with the holidays that can be grown outside with some protection.
Upcoming Gardening Events in December
While some gardening events are winding down this month, there are still several gardening classes and events throughout December. You’ll find an up-to-date list of gardening-related events in our Tucson Gardening Events Calendar.
Don’t live in Tucson? Look for Zoom classes. You can participate in these no matter where you live.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for 15 years.
