September Gardening
Things will be heating up soon enough, so enjoy the cool mornings and breezy evenings while they last! There’s no shortage of things to do in the garden this month, but don’t let it overwhelm you. Many of this month’s chores revolve around completing winter clean-up and preparing for the summer. Some plants need a dose of fertilizer to keep them growing strong, insect populations need to be monitored, and watering schedules should be evaluated to make sure your plants are getting enough water as the temperatures continue to rise.
What to Plant in April
- Annual seed: Coreopsis and sunflower seed can still be planted until mid-month
- Annual transplants: aster, coneflower, cosmos, creeping zinnia, gaillardia, lisianthus, pentas, portulaca, vinca, zinnia
- Bulbs: crinum lily, habranthus, rain lily, and spider lily
- Warm-season succulents and cacti
- Watch newly planted cactus and succulents for sunburn, and protect them from southern and western sun exposure through the first summer if necessary
- Water newly planted succulents in the ground every 7-10 days for the first month, or until new leaves emerge, and then water every 10-14 days through the first summer
- Citrus trees
- Protect the bark of smaller trees from sunburn for the first year or two if necessary, or until the tree has enough leaves and branches to shade the trunk
- Figs, grapes, and deciduous fruit trees
- Summer-flowering perennials
- Rooted, containerized roses
- Frost tender and summer-flowering shrubs
- Evergreen and cold-hardy shrubs can also be planted this month, but they may need more frequent watering and some protection from the hot afternoon sun through the summer to get them well-established.
- Desert-adapted/native trees, and any trees that are frost-tender
- Plant lawngrass from seed, plugs, or sod once nighttime temperatures are regularly above 65-70°F
- Veggie transplants: black eyed peas, cantaloupe, carrots, cucumber, eggplant, Jerusalem artichoke, jicama, lima beans, melons, muskmelon, okra, peanuts, pepper, pumpkin, radish, snap beans, summer squash, tomato, watermelon
- Herbs: basil, bay, chives, curry, garlic chives, hyssop, lemon balm, lemongrass, marjoram, Mexican oregano, oregano, rue, santolina, scented geraniums, spearmint, tansy, thyme, winter or summer savory, yarrow
What NOT to Plant in April
- cool season succulents
- palm trees can be planted this month if temperatures are warmer than usual, but if nighttime temperatures are still cool it might be better to wait until the heat of summer really sets in. Palms love the heat, and will grow and root faster in warmer weather.
What to Fertilize
- Potted annuals can be fed every other week, annuals in the ground can be fertilized every 2-4 weeks with a slow-release or dry organic fertilizer
- Feed iris monthly while it is actively growing, and fertilize summer-flowering bulbs planted in the ground once this month if you didn’t do so last month
- Warm-season succulents in containers can be fed once this month with a water soluble fertilizer mixed at half the rate recommended for houseplants
- Strawberries
- Fertilize newly-planted grapes that have grown more than 6 inches lightly (1/2 oz of nitrogen is recommended in spring and fall of the first year)
- Next year, feed them with twice the amount of fertilizer every month from Feb-Jun
- Every year after that, feed grapes with ¾ lb of nitrogen in February, and ¼ lb of nitrogen in May
- To calculate the amount of any given fertilizer you need to get the desired amount of nitrogen, divide the ounces of nitrogen you want by the percentage of nitrogen in the fertilizer (remember that 1 lb = 16 oz)
- To find the percentage of nitrogen in the fertilizer, divide the first of the 3 numbers on the fertilizer’s label by 100
- Containerized ornamental grasses: once a month through their growing season
- Ornamental grasses in the ground can be fed once this month (they shouldn’t need to be fed again until next April)
- Fertilize lawn grasses only after they have started to actively grow
- Feed perennial and shrub beds with a well-balanced, all purpose plant food at a rate of 1lb per 100 square feet
- Rose growers have differing opinions about exactly when and how to fertilize roses, but some growers use soluble fertilizer every two weeks while roses are blooming, and switch to a granular fertilizer for the rest of the year
- Fertilize established non-native trees once this month (if you didn’t do so in March) with a well-balanced granular fertilizer at a rate of 1 lb per 100 square feet of coverage.
- Don’t fertilize palms this month
- Hold off on fertilizing winter-growing herbs like lavender, marjoram, rosemary, sage, and thyme until fall
- Fertilize leafy veggies with a liquid organic fertilizer or fish emulsion every 2 weeks while they’re growing
- Fertilize root and bulb veggies once at the beginning of the growing season and then again halfway through the growing season.
- Good green tops but poor root systems on root or bulb veggies is usually a sign that they’ve been over-fertilized
- Fertilize peppers and tomatoes once this month, but be careful not to over-apply nitrogen, as this will weaken fruit production in favor of leafy growth
Prune, Divide, and Conquer
- Finish pruning frost damage out of trees and shrubs by the end of the month, if possible
- Deadhead spring-blooming plants for repeat flowering
- Cut back spring flowering perennials and shrubs after they’re done flowering
- The general rule of thumb is to take no more than 25% of the plant’s foliage
- Fall-flowering shrubs can be lightly pruned if necessary, but save heavy pruning for when they’re done blooming in fall
- Leave bulb leaves on the plant until they dry up completely and can be easily removed with a gentle tug – allow them to gather as much energy as possible while any part of the leaf is still green.
- Cut back warm-season succulents and cacti that have grown too large and/or remove any freeze damage if you didn’t do so last month
- Make cuts at joints when possible, and treat cuts with dusting sulfur to help them dry and prevent fungal infection
- If you want to enjoy the flowers, wait until chollas and prickly pears have finished blooming before cutting them back
- Dethatch bermuda lawns any time now through early summer, most other lawn grasses should be dethatched by mid-May
- Unless you’re saving seed, remove the flowering stalks from your penstemon plants after the flowers are all bloomed out. This will force the plant to store energy rather than putting it into seed production.
- Deadhead roses to keep the blooms coming
- Tips for deadheading roses can be found here
- Prune off watersprouts and any stems emerging from below the graft on your fruit trees
- Don’t prune palms or legume trees this month, other than to remove dead or dangerous limbs or branches
- Take cuttings of bay, lemon verbena, marjoram and mint
- Cut back any summer-growing herbs, such as basil, lemongrass, and mint, if they’ve become too overgrown
- Don’t prune woody winter-growing herbs like rosemary, sage, lavender, or thyme until fall
For the Fruits
- Clean up any citrus fruit that prematurely drops from the tree (some premature fruit drop is normal as temperatures and winds increase this month)
- Fertilize newly-planted grapes that have grown more than 6 inches lightly (only 1/2 oz of nitrogen is recommended in spring and fall of the first year)
- Next year, feed them with twice the amount of fertilizer every month from Feb-Jun
- Every year after that, feed grapes with ¾ lb of nitrogen in February, and ¼ lb of nitrogen in May
- To calculate the amount of fertilizer you need for the desired amount of nitrogen, divide the ounces of nitrogen you want by the percentage of nitrogen in the fertilizer (remember that 1 lb = 16 oz)
- To find the percentage of nitrogen in your fertilizer, divide the first of the 3 numbers on the fertilizer’s label by 100
- Cover fruiting trees with bird netting as the fruit begins to grow. This will keep birds from ruining the fruit before it ripens.
- Prune off watersprouts and any stems emerging from below the graft on your fruit trees
For the Lawn
- Plant lawngrass from seed, plugs, or sod once nighttime temperatures are regularly above 65-70°F
- Water established bermuda lawns to a depth of 8-10 inches every 8-10 days while it is dormant, but step up watering to weekly once the grass begins to grow
- Fertilize lawn grasses only after they have started to actively grow
- Dethatch bermuda lawns any time now through early summer, other lawns should be dethatched by mid-May
Water Wisely
- It’s time for an irrigation check-up! Check for proper functioning of your timer, and make sure that emitter lines aren’t clogged, pinched or missing parts. Add emitters where necessary if trees or shrubs have grown larger. Flush out your poly lines, clean out filters, and check for leaks.
- Water annuals to a depth of 6-8 inches (to 1 foot for larger plants)
- Be prepared to water container plants every other day, or daily if temperatures and/or winds cause potted plants to wilt
- Move the container to a spot with more shade if the plant is wilting too frequently
- Cut back on watering frequency of winter-growing succulents by half when the leaves begin to yellow, and stop watering completely when the leaves fall off
- Water newly planted succulents that are in the ground every 7-10 days for the first month, or until new leaves emerge, and then water every 10-14 days after that through the first summer
- Established warm-season succulents need water every 2-3 weeks until temperatures are steadily above 105, at which point they need water every other week. Very large native succulents that are already established need water monthly through the hottest summer months
- Established deciduous fruit trees: every 7-10 days to a depth of 3 feet
- Established citrus: every 10-14 days to a depth of 3 feet
- Established bermuda lawns: to a depth of 8-10 inches every 8-10 days while it is dormant, but step up watering to weekly once the grass begins to grow
- Established native perennials: to a depth of 1-2 feet (depending on the size of the plant) about every 3 weeks
- Non-native perennials: to a depth of 1-2 feet (depending on the size of the plant) and water frequently enough to maintain moisture at that depth, which might mean weekly watering when the temperatures are high
- Roses: to a depth of 2 feet frequently enough to maintain moisture at that depth, which might mean weekly watering when the temperatures are high
- Established non-native shrubs: to a depth of 2-3 feet (depending on the size of the shrub) around every 1-2 weeks, depending on winds and temperatures. Water just frequently enough to maintain moisture at a depth of 2-3 feet through the summer.
- Native and drought-tolerant shrubs: every 3-4 weeks to a depth of 2-3 feet (depending on the size of the shrub)
- Palms: every other week to a depth of 3 feet
- Young trees: to a depth of 3 feet every 7-10 days, or as often as water is needed to maintain moisture at that depth
- Established trees need varying amounts of water, depending on the species and how deep the tree’s roots are. The general rule of thumb is to water just frequently enough to maintain moisture at a depth of 3 feet.
General Chores
- Harvest cool-season annual seed when the seeds start drying out or turning brown
- Treat agaves and yuccas preventatively for boring insects with a systemic insecticide if they are a known problem in your garden
- Give rose bushes regular showers in the early morning hours to stave off powdery mildew and spider mites.
- Aphids can be hit with a hard spray of water, treated with insecticidal soap, or picked off by hand if you catch them early and populations are still low. You can also pick up some ladybugs or praying mantis eggs at the garden center; release them in your garden at the base of infested plants and watch them go to town!
- Watch for webbing on the new growth of Texas Mountain Laurel; it is made by the caterpillars of the Genista Moth, and they are most easily controlled by pruning them out of the tree and destroying them.
- Apply a 3-4 inch layer of mulch (straw, leaf litter, or clippings) over veggie beds to conserve moisture and help keep roots cooler
- Avoid using fresh grass clippings, add them to your compost instead
- If you get blossom end rot or cat-facing on tomatoes, carefully monitor you watering schedule to make sure that tomatoes are watered deeply and regularly, but not too frequently. Add a 30-50% shade cloth over tomatoes when temperatures are above 100°F if sunburn is a problem.
- Watch rosemary, sage, and other perennials for a frothy foam to show up on the lower branches. This is left by an insect called spittlebug, and it can be washed off of the plant easily with a spray from the hose.
- Leftover eggshells don’t have to go in the trash! Check out this post about using eggshells in the garden and beyond.
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
