September Garden Checklist
Learn what to plant this month and get landscape maintenance advice for all Arizona elevations.
By Kelly Young
What to Plant
September marks the beginning of the fall planting season in the low desert. Cooler temperatures reduce the risk of plant—and human—stress from extreme heat and intense sunlight. New plantings establish a strong root system and thrive once daytime temperatures max out at approximately 100 degrees, usually around midmonth.
COOL-SEASON VEGETABLES–Leafy greens, including kale, spinach, arugula, chard and leaf lettuce; root vegetables, such as carrots, beets, turnips and radishes; and sugar and snap peas can be planted from seed now. Provide a trellis for peas to grow vertically and save space in the garden. Follow the recommendations on the seed packet for planting depth and to maintain optimal spacing. Crops that take longer to mature, such as broccoli, cauliflower, head lettuce, cabbage and Brussels sprouts, should be started from transplants to give them enough time to develop before next spring.
WILDFLOWERS–Sow seeds of wildflowers now for a resplendent show of blossoms in late winter. Scatter seeds over soil that has been cleared of weeds and gently water to encourage them to settle and make contact with the soil. Cover the seeds with a blanket of mulch, no more than one-half inch deep. This keeps the seeds from being blown away but will not impede germination. Globemallow (Sphaeralcea ambigua) is a native perennial that blooms in shades of orange, pink, purple and white. Blanketflower (Gaillardia aristata) produces daisylike blossoms with bands of red, orange and yellow.
TREES, SHRUBS AND VINES–For a fast-growing, deciduous tree that produces striking red flowers that attract hummingbirds to the yard, try coral bean (Erythrina bidwillii). San Marcos hibiscus (Gossypium harknessii) is a yellow-flowered shrub native to northern Mexico that thrives in the hottest, driest environments with minimal care. To create a lush, tropical feel in your garden, give pink trumpet vine (Podranea ricasoliana) a place to twine along a post or trellis. Dig a hole only as deep as the plant’s roots in the pot but at least twice as wide. If the hole is too deep, the plant stem can become weakened from exposure to moist soil. If it is too narrow, roots may have difficulty colonizing their new habitat.
GARLIC–If garden space is limited or you’re working with containers, garlic is a great option, because each plant only requires an area 6 inches deep and wide to be productive. Select large, plump cloves from grocery store produce or from a nursery supplier that are free from damage and plant them so the flat end is down and the pointed end is approximately 2 inches below the soil surface. Cover with a 3- to 4-inch-deep layer of mulch, such as straw or compost, to insulate the soil and conserve moisture. Garlic bulbs will be ready to harvest early next summer.
Middle and High Elevations
ORNAMENTAL TREES AND SHRUBS–Enhance the beauty of your landscape and add value to your home with varieties that thrive at your elevation with minimal care. New Mexico locust (Robinia neomexicana) is a native, deciduous tree that blooms pink in the spring and tops out at 25 feet in height. For a large shade tree, Douglas fir (Pseudotsuga menziesii) is evergreen and reaches 50 feet tall at maturity. Blue mist spirea (Caryopteris clandonensis), a low water-use shrub, produces purple flowers in late summer that are attractive to bees and butterflies. When selecting a site for new plants that are intended to be permanent installations in your landscape, ensure that the space offers ample room to grow without limbs interfering with buildings, driveways or fences. Chat with your nursery professional to learn how large you can expect your new purchase to grow.
ADJUST IRRIGATION–Most plants do not need to be watered as frequently in the fall as they do in summer. Gradually increase the number of days between irrigations over the next several weeks. For example, if you were watering your citrus weekly during the summer, by mid-November you should only be watering once every two weeks.
CLEAR WEEDS–Remove weeds that crept into your garden and landscape over the summer. Perennial types that grow back from underground stems year after year, such as buffelgrass (Pennisetum ciliare), should be dug out to prevent them from resprouting next year. Promptly dispose of weeds that have gone to seed and avoid dragging them around, thereby accidentally spreading seeds throughout your yard.
PREP FALL BEDS–Enrich the soil in flower and vegetable beds to prepare for planting in October. Add 3 to 4 inches of compost and work into the upper 8 to 10 inches of soil. Compost will gradually release nutrients that are essential to plant growth and reduce the need for supplemental fertilizer. You can amend compost with 5 to 6 pounds of ammonium sulfate per 1,000 square feet of garden area to support vigorous crop growth.
How do I know when it’s time to kill the cowpea cover crop in my vegetable garden? I’m anxious to get ready for fall planting, but I don’t want to undermine my efforts by being impatient.
Cover crops are planted to blanket the soil rather than for the purpose of being harvested. The timing of “terminating” or killing cover crops at the end of their usefulness depends on the development of the cover crop itself and the planting window for fall vegetables. Cowpeas (Vigna unguiculata) are usually terminated once approximately one quarter of the plants have started blooming. If you wait much longer, they may begin locking up nitrogen in their seeds, making it unavailable in the soil. Since fall planting kicks off later this month, I’d recommend terminating the cover crops now, regardless of the cowpea’s developmental stage. Chop them down, lightly mix them into the soil, and add enough water to penetrate about 1 foot deep to encourage decomposition by bacteria and release of nutrients, which may take about a month. At that time, you are ready to plant your fall vegetables.
Our basil is blooming. Do we need to dig it out and start new?
Unlike some herbs, such as cilantro and dill, which die immediately after blooming and setting seeds, basil continues to be harvestable while in flower. In fact, you can eat basil blooms, which taste similar to the leaves. You can also cut the blossoms off as they appear and add them to cut flower arrangements. Basil flowers are attractive to honeybees, so you might consider leaving them in place for the sake of pollinators.
The leaves on my hibiscus are yellow, and the plant seems to be struggling overall. I’ve tried fertilizing with special hibiscus food, but it doesn’t seem to be helping. What could be the problem?
Long, dry summers can take a toll on tropical beauties, such as hibiscus. Yellow leaves can indicate a number of maladies, including nitrogen deficiency, which you have already addressed, and irrigation problems—both too much or not enough. Check the soil moisture around the roots, about 3-4 inches below the surface, to see if it is either bone dry or saturated with water and adjust irrigation accordingly. The soil should be damp but not waterlogged.
Hibiscus are also a favorite of whiteflies, and unhealthy plants can signal an infestation. Give plants a shake and watch for a swarm of the tiny winged insects to appear. If it does, know that treatment options are limited. You can apply a systemic insecticide labeled specifically for managing whiteflies on hibiscus. Follow the instructions exactly to avoid accidental damage to other creatures or mammals that visit your landscape. Alternatively, you can try waiting the pests out. Most whiteflies will disappear in another month. Do your best to minimize water stress and avoid pruning until next spring. Stressed plants and new shoots, which will grow as a result of pruning, are most vulnerable to damage by whiteflies.