May: What to Do in the Desert Garden

As the weather heats up in May, gardening activity winds down. Discover what you can still plant, prune, and fertilize (and what you shouldn’t), how to adjust your watering schedule as you head into summer, and more.

May is a beautiful month in the desert. Plants are flowering and bursting with new spring growth. But as the weather heats up, gardening slows down. (Or so it should, if you’re doing it right!)

While there are still plenty of gardening activities to be done in May, there are some tasks that you should skip for now — they are best done in cooler weather.

Let’s take a look at what you should and shouldn’t plant, prune, and fertilize, how to adjust your watering schedule, insects and weeds to watch out for (look for two good guys this month!), and more.

May Weather Outlook

Compared to April, May is noticeably hotter, drier, and harder on plants (and gardeners). The average high temperature is nearly 10 degrees higher. The chance of rain goes down even further, dry winds pick up, and the sun’s intensity increases.

Here’s a look at May weather averages in both Tucson and Phoenix.

May Weather Averages (Tucson)

High temperature	92°F
Low temperature	61°F
Precipitation	.24″
Relative humidity	18%
UV Index	10.4
Wind	15 mph

May Weather Averages (Phoenix)

High temperature	95°F
Low temperature	69°F
Precipitation	.13″
Relative humidity	18%
UV Index	9.3
Wind	8 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.

What To Do in the Garden in May

Here’s a snapshot of the tasks you can do this month and those you should stop doing.

Read on for all the details and exceptions!

What You Should (& Shouldn’t) Plant in May

The list of what you should plant in May is a small one.

Some plants should be planted in May. It’s an ideal time to plant cactus, agaves, and other succulents since warm soil temperatures encourage root development. You can move agave pups and divide clusters of aloe now as well.

May is the perfect time to plant summer annuals such as gaillardia, marigold, pentas, moss roses, sweet potato vine, vinca, and zinnia.

While you “can” continue to plant native or desert-adapted trees, shrubs, ornamental grasses, and perennials, planting now is not ideal and I do not recommend it.

There’s not enough time for plants to get acclimated and establish new roots, and summer is just too darn stressful for new plants.

But if you do plant in May, be sure to give your new plants some extra TLC. Learn the steps you can take to minimize planting stress and get your plants off to the best possible start in our article 6 Steps for Successful Spring Planting.

Adjust Your Watering Schedule Mid-Month

If you look at various watering charts, you’ll find that a spring is considered “March through May” while summer is considered “May through September”. I suggest adjusting your irrigation to a summer schedule in mid-May.

As the weather heats up, it’s important that you’re watering correctly. Use the University of Arizona’s Recommended Watering Schedule as a simple guide or Landscape Watering by the Numbers flipbook as an in-depth guide.

For more help on how to water your plants in May,
check out these articles:
How & Why to “Deep Water” Desert Plants
How to Water Desert Trees: How Often? How Much?
How to Water Desert Shrubs: How Often? How Much?

What to Fertilize

May is the best month to fertilize palms. They need warm soil to absorb nutrients. Continue to fertilize potted succulents and annuals every few weeks.

Orange and grapefruit trees can receive their third dose of fertilizer this month. The rule of thumb used to be to fertilize three times per year at the holidays — Valentine’s Day, Memorial Day, and Labor Day — but this schedule has been updated.

Here’s the University of Arizona’s most recent citrus fertilization calendar. Dates are now a little earlier and have a range, rather than being tied to specific holidays.

	Lemons, Limes	Oranges, Grapefruit
1st	Jan. – Feb.	Jan. – Feb.
2nd	March – April	March – April
3rd	Aug. – Sept.	May – June

You can easily determine how much fertilizer to apply with University of Arizona’s handy online citrus fertilizer calculator.

While we’re talking about citrus, don’t be concerned if your trees lose older yellow leaves or drop premature fruit this month. Your trees are smart — they only keep the amount of fruit that they can comfortably support.

What to Prune

Other than cactus and other succulents which prefer to be pruned when it’s hot, now is not the time to do much pruning.

You can lightly prune trees and shrubs to shape or remove the 5 Ds — dead, diseased, damaged, dangerous, or deranged branches.

You can now prune spring-flowering shrubs that have finished blooming (i.e., fairy dusters, emu bushes), or remove spent flower stalks from perennials.

You can continue to deadhead annuals and perennials to encourage flowering.

Harvest Wildflower Seeds

Some wildflowers such as desert bluebells, penstemon, and native poppies may have finished flowering. Look for seeds on the dried flower stalks. If you have any growing on your property, you can harvest seeds now to sow next fall.

See These Insects? Let Them ‘Bee’

With warmer weather comes increased insect activity. Resist the impulse to kill bugs you see on your plants until you know what they are — some of them are the good guys! Here are two kinds of insects that make their presence known this month that you don’t want to harm.

Leaf Cutter Bees

You may notice circular cutouts on the leaves of some of your plants, especially roses, redbuds, and bougainvillea. These are caused by leaf cutter bees.

Leaf cutter bees are non-aggressive, solitary bees that are important pollinators, so please do not kill or discourage them.

Any damage you see is purely cosmetic and won’t harm your plant. Leaf cutter bees are quite amazing. They cut plant circles and wrap up their larvae, making “baby bee burritos.” 🙂

Swallowtail Larvae

If you see something on your citrus leaves that looks like blob of bird poop, please leave it be. It’s the larvae of the beautiful giant swallowtail (Papilio cresphontes) and will only cause minor cosmetic damage. This is one of the largest butterflies in North America — they can be up to 6″ across! — and we are fortunate to have them here.

If you want butterflies, you gotta put up with a little poop! 🤗

Watch for Weeds (especially stinknet)

Depending on how much rain you’ve had, you still may have some weeds pop up this month. If you sowed wildflower seeds in the fall or hope some of your plants will reseed, be sure you aren’t pulling any seedlings of plants you want in your yard.

If you live in Arizona or southern California, continue to keep an eye out for stinknet, an alarmingly aggressive, invasive plant that threatens desert biodiversity, contributes to wildfires, and emits toxic gases when burned.

If you aren’t sure whether stinknet has made it to your area, check this interactive map.

Every desert gardener should know how to identify it and how to get rid of it safely. To learn more, watch this video or visit Stinknet.org.

Upcoming Gardening Events in May

There are still some gardening classes and events throughout May but activity is winding down. 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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Image Credits

“Megachile Centuncularis” by JRxpo is licensed under CC BY 2.0