Tips from the experts at Desert Botanical Gardens
By Tracy Rhodes, Gardener Il
As summer heat gives way to autumn coolness, gardeners look forward to a time of renewal, a second spring of sorts. The mild months ahead means myriad planting possibilities.
For those who prefer to harvest what they plant, fall is prime time for cultivating a vegetable and herb garden that will pay dividends well into spring.
Preparing your fall garden
Planting time depends not so much on specific dates as on weather conditions over several critical weeks. Gardeners should watch long-range forecasts and observe their garden’s sun exposures.
Cool season plants need at least four to six hours daily of direct sunlight. By the time high temperatures drop into the 90s and lows into the 70s, the season has begun.
Prepare soil by turning over the top six to 12 inches, removing old plants and roots, and adding amendments. Compost is a common soil amendment, along with aged manure (pelletized chicken manure is the Center for Desert Living’s choice, for its trace minerals and slow-release properties). Compost and manure not only loosen and nourish the soil but help bring its pH into the slightly acidic range preferred by most vegetable plants.
Supplemental fertilization also is important, especially if you have just grown heavy-feeding summer crops.
All-purpose vegetable fertilizers provide nitrogen, phosphorus and potassium (N-P-K) and often other nutrients such as calcium, sulfur, magnesium, boron and iron. Rich soil promotes growth of beneficial soil bacteria, fungi and insects. Adding amendments and fertilizer a few weeks before planting will allow them time to integrate with the soil.
Gardeners challenged by a small yard, hardpan soil or mobility issues may want to consider using raised beds or pots. Raised beds offer easy access and the opportunity to customize soil content. They should be large enough to be useful and deep enough to support long roots: 4 feet by 8 feet by 18 inches are typical dimensions.
Construction materials may vary from untreated redwood to concrete blocks or even hay bales. When filling beds, native soil is useful for its mineral content, but limit it to 30-50% of bed composition, which is preferable, combined with richer, water-wise components such as compost, manure, coir, peat, sand, gypsum or crushed lava rock. Alternatively, most fall herbs and vegetables should grow in pots with good drainage. The deeper the pot, the better; ceramic or clay will insulate roots better than plastics.
Selecting the seeds
Many crops thrive in desert winter months, including dozens of leafy greens, carrots, radishes, beets and beets’ leafy cousin, Swiss chard.
Consider plants from the brassica family, including cabbage, broccoli, Brussels sprouts, kohlrabi, cauliflower, mustard greens and kale. Peas, fennel, celery, onions, and many cooking herbs such as parsley also thrive at this time of year.
In January, plant seed potatoes for a late spring harvest.
A key consideration in crop selection is a characteristic known as “days to maturity.” DTM represents the approximate time between when a plant begins its mature growth – marked by true leaves sprouting after the initial seed or cotyledon leaves emerge – and when its food portions may be harvested. For transplants, DTM is typically counted from date of transplant.
Crops with a short DTM range of 40-65 days, such as radishes, beets and lettuces, may be planted more than once throughout the growing season for constant availability, a process known as staggered or succession planting.
Most leafy greens will yield multiple harvests with careful exterior picking; just leave an inner leaf core that will continue to produce new growth.
Tips to keep in mind
Planting calendars specific to Phoenix can be invaluable guides. Two reliable calendars for the Valley are the University of Arizona Extension’s Vegetable Planting Calendar (extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1005.pdf), and Urban Farm’s “Low Desert Planting Calendar” (http://www.yourguidetogreen.com/TheUrbanFarm/wp-content/uploads/Planting_Calendar.pdf).
Both guides provide optimal planting time frames and growing tips such as cold tolerance, whether to start seeds indoors or directly in beds, and how soon to expect harvest. Additional resources can be found in the Desert Botanical Garden’s Schilling Library.
Plants with longer DTMs of 75-110 days, such as cabbages, must be started as soon as possible so they can mature before spring warmth causes bolting. Indoor germination using greenhouses, heat mats, grow lights, or even a sunny window can help gardeners get a jump on the weather. Aside from root crops, most cool season seedlings transplant well.
Direct-sown seeds such as squashes may be pre-sprouted in a damp paper towel before planting. Garden soil should be damp when planting and remain so while seedlings develop.
Adding several inches of wood mulch around young plants conserves water, controls weeds and insulates from cold. Watering should be deep but staggered; constantly soggy soil can sometimes drown plants. A soil moisture monitor can be handy for checking watering needs. Usually one or two deep soaks per week are sufficient. Regarding pests, flea beetles may leave tiny but mostly harmless buckshot-sized holes in leaves. Aphids are tenacious but may be removed with water or soap sprays. Looper worms and beetles are easily hand-plucked. A real benefit of cool season gardening is that plants encounter fewer pest, weed and weather stressors.
The majority of established cool weather veggies will survive light frost without covers. In fact, some growers claim that winter crops gain sweetness from exposure to a light freeze. Even lettuces that seem frost-melted should be left alone; by afternoon they are often good as new. Most losses will be from seedlings or leftover warm season vegetables such as basil, tomatoes and peppers.
With care, cool season gardens can thrive and offer vitamin-rich fresh produce well into the following spring. Some herbs and hardy greens such as Swiss chard may even survive through summer and become perennial producers. Fall is a great time to experiment with what works in your garden.
This article originally appeared in The Sonoran Quarterly, September 2015, Volume 69, No. 3. Photos coutesy of Desert Botanical Gardens.
