September: What to Do in the Desert Garden

While September might mean fall elsewhere, it’s still summer here in the desert Southwest. However, the monsoon is winding down, which brings some changes in weather and gardening tasks. Here’s a look at what you should and shouldn’t do in the garden now, and learn why September is the month to plan, not plant.

Summer is winding down and the monsoon officially ends at the end of the month. As the average temperature cools down slightly, garden activity ramps up. After months of merely trying to keep your plants alive, plants and gardeners experience a little relief this month.

September Weather Outlook

Even though it may feel like fall will never arrive, there is some progress on the weather front. September is a little cooler and less humid than August. The average evening low in Tucson is a delightful 70°F… nice! 😉 The days are getting noticeably shorter, and the sun continues to get less intense. But the downside is that average rainfall is significantly less in September than in previous months.

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

September Weather Averages (Tucson)

High temperature	95°F
Low temperature	70°F
Precipitation	1.28″
Relative humidity	39%
UV Index	9.0
Wind	14 mph

September Weather Averages (Phoenix)

High temperature	101°F
Low temperature	78°F
Precipitation	.59″
Relative humidity	29%
UV Index	8.2
Wind	7 mph

If you live elsewhere, enter your city or town in the search bar at TimeandDate.com to find your monthly averages. You can find your average UV index at UVIndex.io.

What To Do in the Garden in September

As you’ve done all summer, continue to keep your plants hydrated, pull weeds, and watch for pests.

Here’s a snapshot of the activities you can do this month and those you should skip.

✅	Plan for fall planting
✅	Water as needed
✅	Protect any new plants
✅	Fertilize lemons & limes
✅	Prune storm damage
✅	Prune perennials
✅	Watch for insect pests
✅	Harvest prickly pear fruit
❌	Don’t plant most plants
❌	Don’t fertilize most plants
❌	Don’t prune most plants

Read on for all the details and exceptions!

Should You Plant in September? The Case for Waiting

Many gardening references state that September is a good time to plant trees, shrubs, perennials, and winter annuals.

But I disagree.

Many of these recommendations were written years ago, and things have changed. The past few Septembers have been brutally hot. In September of 2024, here in Tucson, we had 16 days over 100. October, with 13 days over 100, brought little relief. That’s two months of extreme heat for new plants.

I suggest waiting until mid-October or even November to plant. Your plants will experience less transplant shock, and you’ll experience less stress.

You can plant cactus, agave, yuccas, and other landscape succulents this month, but if you can wait until spring, April is considered “prime time” for planting succulents. If you do plant them now, protect them from full sun until the weather turns cooler and the sun is less intense. (That may be until November.)

If you get the overwhelming urge to plant or have a plant that needs to go in the ground ASAP, you can, but you’ll need to be vigilant about watering and protecting it from the sun. After a long, hot summer, I’m burned out trying to water and protect my plants.

Plan, Don’t Plant

This month is the perfect time to plan (not plant!) what you want to plant later this fall. October is the best overall month for buying and planting landscape plants. November is a great time to plant, too, but nursery stock can run low.

The end of the summer is the perfect time to do a “post-mortem” of your garden, so that you don’t repeat the same mistakes.

• Are there any plants that didn’t make it through the summer and need to be replaced?
• Are there any plants that caused you more stress than joy? Maybe it’s time to take them out and replace them with plants better suited to their environment.
• Are there any bare spots in your yard beckoning for a new plant?
• Do you wish you had more color in your yard in the fall or winter? Decide what to plant for colorful fall leaves or flowers that bloom in the winter, so you can enjoy your garden more.

Before deciding on any new plant, make sure it’s a good choice for the spot you have in mind. Choosing the right plant can save you time, money, and disappointment. Here are the top factors to consider before making any major purchases.

If you’re in the market for a new tree and don’t mind starting small, check out Tucson’s Trees for You Program which starts this month. Tucson Electric Power (TEP) customers can order two 5-gallon trees for $5 each.

You can learn more about the trees they offer and whether they’d be a good fit in your yard in our article TEP’s “Trees for You”: Choosing the Right Tree.

And you can find comprehensive “Growing Guides” for over 200 popular landscape plants in our Desert Plant Library. This information is invaluable for finding the perfect plant for any spot.

Get ready to sow wildflower seeds in October!
Find sources of wildflower seeds here.

How About Cold Weather Annuals?

A visit to most nurseries this time of year will wow you with eye-popping displays of colorful annuals. If nurseries are selling them, this must be a good time to plant, right?

Unfortunately, the answer is “no.”

Nurseries put out annuals well before the best time to plant them. Marylee Pangman, Tucson’s leading authority on potted plants and author of Getting Potted in the Desert, recommends waiting until your nighttime temperatures are consistently in the mid-fifties before planting winter annuals.

She’s found through years of trial and error that waiting for cool nights will reward you with happier, healthier, more beautiful plants. That means waiting until October or even November to plant cold season annuals, depending on the species.

However, there are a handful of “shoulder season” annuals you can plant towards the end of the month, provided the weather has not been unseasonably hot. Shoulder season annuals include osteospermum, dianthus, dusty miller, petunia, and snapdragon.

Provide New Plants with Shade

If you do plant anything new this month, provide it with temporary shade.

Additionally, if you lost a tree or a large part of a tree due to storm damage, you may have plants that have suddenly lost their shade. This abrupt change can fry them.

You can provide temporary shade with shade cloth or something as simple as a patio chair. If you have fallen branches, you can use them to temporarily shade new plants. Patio umbrellas work great, but don’t leave them up unattended. Sudden winds can knock them over or turn them inside out.

Also, be mindful that the position of the sun is changing. Relocate any potted plants as needed to account for the movement of the sun.

Watering

September generally gets less rain than the previous months and continues to be hot, so continue to water on a summer watering schedule.

Not sure how to water your plants in September?
Our Desert Watering Resources can help.

What to Fertilize

Most plants should not be fertilized this month, but there are a few exceptions.

Fertilize Lemons & Limes

Citrus should be fertilized three times per year. If you didn’t fertilize your lemons or limes in August, fertilize them early in September. This will be their third (and final) application for the year.

(Other citrus — oranges, tangerines, grapefruit — should not be fertilized now. They should have had their final application in the spring.)

Fertilizing instructions can be complicated. How much fertilizer to use depends on the size of your tree and the amount of nitrogen (N) in your fertilizer. To determine exactly how much fertilizer your trees need, use the University of Arizona’s online Citrus Fertilizer Calculator.

Fertilize Container Plants

Potted annuals, succulents, or any other plants growing in containers should be fertilized this month. Plants in pots have limited access to nutritional resources and can eventually use up all the nutrients in the potting soil.

What to Prune

There are a few pruning tasks you can do this month.

If you have any large cactus or trees that were uprooted by storm damage or are leaning precariously, call a cactus specialist or an arborist. They might be able to reposition and save your plant.

You can remove the 5 Ds — dead, diseased, deranged, damaged, or dangerous branches — from shrubs or trees.

You can trim any leggy perennials ideally once the temperature stays below 100°F.

You can deadhead spent flowers anytime.

Watch for Garden Pests

It’s not unusual to see an uptick in insect pest activity this month. Here are some common things to watch for.

If you see something that looks like bird poop on your citrus, leave it alone. It’s the larval stage of the beautiful giant swallowtail butterfly and won’t hurt your tree.

Whiteflies and spider mites thrive in hot, dry weather. If you see them on your plants, give them a spray with the jet setting of your hose nozzle. If that doesn’t work, you can try insecticidal soap, but only when the temperature is below 90. Using soap, neem oil, or any other spray for insects in the heat can burn your plants’ leaves.

If you notice white fuzzy cochineal on your prickly pears, hose it off.

If any of your agaves suddenly look yellow or droopy, it may have agave weevils. Give a plant a gentle rock — if it’s infected, it will easily move and even break off at the base. Learn what to do about agave weevils here.

If you see small caterpillars dining on your Texas mountain laurel’s leaves, there are several approaches you can take that range from picking them off by hand, to spraying or letting them do their thing.

Harvest Prickly Pear Fruit

August through September is the time to harvest ripe prickly pear fruit. If you still have fruit to harvest and prepare, the Arizona-Sonora Desert Museum’s article “Prickly Pear Harvesting 101” will tell you everything you need to know.

Upcoming Gardening Events in September

Tucson always has a lively gardening scene, and this month you can look forward to a growing number of gardening-related activities since some nurseries and organizations pause events during the summer.

Most weeks there are multiple garden-related events — classes, workshops, plant sales, musical events, food trucks, garden-related crafts, and more. There’s always a mix of both indoor and outdoor activities.

Don’t live in Tucson? Look for classes being held online. You can participate in these no matter where you live.

You’ll find a comprehensive list of nurseries, botanical gardens, and organizations that hold classes, events, and sales in our Tucson Gardening Event Guide.

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