September Gardening
August is often a wet month in Southern Arizona, but don’t pack away your garden hose just yet. It’s time to get our hot and sunny gardens ready for the upcoming fall frenzy!
What to plant in August
- Warm-season annuals like sunflower (from seed or starts), vinca, and portulaca
- Seed of spring-flowering bulbs
- Warm-season cacti and succulents
- Transplant strawberry start this month and in September
- Heat-loving shrubs
- Be sure to provide them with enough water to get established
- Desert trees
- Corn and squash varieties that mature quickly can be planted early in the month
- Cool season veggie seeds can be started inside and transplanted into outdoor beds late in the month
- When temperatures cool down below 100 F, plant beets, bok choy, broccoli, Brussels sprouts, bunching onions, cabbage, carrots, cauliflower, collard greens, cucumber, kale, kohlrabi, lettuce, leeks, mustard greens, peppers, snap beans, Swiss chard, and tomatoes
- Keep these early starts well-watered, and consider giving them some afternoon shade if temperatures are high to give these cool-season plants a better chance of survival until temperatures subside
- Sow seed of herbs late in the month: calendula, chamomile, and chives
What NOT to plant in August
- Anything that goes dormant in the summer heat
- Deciduous trees, shrubs and fruit trees (though they can technically be planted anytime in Tucson, it’s easier to get them established when temperatures are cooler)
What to fertilize
- Potted annuals: once this month
- Annuals that have been in the ground for more than 2 weeks
- Containerized warm-season cacti and succulents: feed lightly once this month with a low-nitrogen fertilizer
- Ornamental grasses: feed once this month with a bloom formula fertilizer
- Warm-season lawn: fertilize according to your regular schedule this month
- Palms
- Citrus trees: feed at the end of the month or in the beginning of September
- Organic fertilizer can be applied around mid-month
What to NOT fertilize
- Anything that has gone dormant in the summer heat and isn’t actively growing
- Avoid fertilizing cold-sensitive plants too late in the year. A spur of new growth just before winter leaves cold-sensitive plants more vulnerable to frost damage.
Prune, Divide, and Conquer
- Deadhead annuals and summer-flowering shrubs or perennials to keep the flowers coming
- Lift out Dutch and Spanish Iris that have gone dormant and store them in a cool, dry spot for 6-8 weeks before replanting them in late September or October
- Divide iris that has been in the ground for 3 or more years
- Remove spent blooming stalks from aloe, hesperaloe, and yucca plants
- Prune prickly pear and cholla to reduce size or to remove parts of the plant that are damaged or diseased
- Chrysanthemums will begin to bud this month, so it’s a good idea to stop pinching them back until after they’ve bloomed
- Perennials that have become too overgrown and leggy can be cut back by 1/3 this month
- Once temperatures have dropped below 90, roses can be pruned lightly to remove damaged or dead branches
- If your roses are showing signs of heat stress, make sure to remove buds before they open into flowers to help the plant conserve its energy. Add some afternoon shade if possible
- Heat-loving shrubs like oleander and bird of paradise can be pruned lightly this month to shape the plant and promote more blooming
- Cut back eggplant and pepper plants that have survived the summer to rejuvenate them for fall fruiting
- Divide mint, garlic chives, and lemongrass that have become too overgrown
For the Fruits and Veggies
- Transplant strawberry starts this month and in September
- Early in the month, plant corn and squash varieties that mature quickly
- Cool season veggie seeds can be started inside and transplanted to the garden later in the month
- When temperatures cool down below 100 F, plant beets, bok choy, broccoli, Brussels sprouts, bunching onions, cabbage, carrots, cauliflower, collard greens, cucumber, kale, kohlrabi, lettuce, leeks, mustard greens, peppers, snap beans, Swiss chard, and tomatoes
- Keep these early starts well-watered, and consider giving them some afternoon shade to give these cool-season plants a better chance of survival until temperatures subside
- Water veggies daily to a depth of about 1 foot, depending on the crop
- Cut back eggplant and pepper plants that have survived the summer to rejuvenate them for fall fruiting
- Fertilize citrus trees at the end of the month or the beginning of September
- Organic fertilizer can be applied around mid-month
For the Lawn
- Water lawns every 4-7 days to a depth of 8-10 inches
- Mow regularly, removing only 1/3 of the blades each time you mow
- Fertilize warm-season lawn on its regular schedule this month
Water Wisely
- Monsoon rains may or may not water plants enough to skip a scheduled irrigation. If we get ½ inch or more of rain you can test your tree or shrub wells with a soil probe to see how deeply the rain watered, and water only if more is needed.
- Annuals and smaller perennials need water to a depth of 1 foot; shrubs, perennials, cacti, agave and yuccas need water down to 2 feet; trees, large shrubs, and larger columnar cacti, agaves, or yuccas need water to a depth of 3 feet.
- Check the berms, basins and ditches around your landscape to make sure that they haven’t been toppled or overrun by heavy rains
- Continue to check potted plants daily for watering needs through the end of the month
- Once this month, water all of your plants twice as deeply as you normally water to move accumulated salts away from their root zones
- Bulbs that are still actively growing: water to a depth of 6 inches
- Large columnar cactus, yuccas and agave: water once this month to a depth of 3 feet
- Smaller cacti, yuccas, and agave: water every 2-3 weeks to a depth of 2 feet
- Trees: water every 1-3 weeks to a depth of 3 feet
- Lawns: water every 4-7 days to a depth of 8-10 inches
- Perennials: water every 3-4 days to a depth of 1 foot
- Roses: water weekly to every two weeks to a depth of 2 feet
- Water as often as necessary to maintain moisture at a depth of 2 feet, frequency varies with weather and soil type
- Shrubs: water to a depth of 2 feet every week or every other week until the heat subsides.
- Water as often as necessary to maintain moisture at a depth of 2 feet, frequency varies with weather and soil type
- Veggies: water daily to a depth of about 1 foot, depending on the crop
- Established herbs: water every 2-5 days to a depth of 1 foot
General Chores
- Begin preparing beds for fall veggies and annuals
- Watch for cochineal scale (which looks like tufts of white cotton) to show up on your cacti and succulents (especially prickly pear). This insect can be easily controlled with a hard spray of water if you check regularly, and catch and treat the outbreaks early.
- Unless the rain has done it for you, give your plants (especially those with glossy leaves) a good shower with the hose at least once a week to reduce dust and help control spider mites
- Remove summer weeds before they make seeds that will sprout into more weeds next year
Sources: University of Arizona online publications, Arizona State University online publications, Desert Botanical Garden website, Tucson Botanical Garden website, Gardening in the Deserts of Arizona by Mary Irish, Arizona Master Gardener Manual
