The low desert areas have two planting seasons for most plants, including vegetable: fall and Spring. Fall is a wonderful time to be planting trees and shrubs that are not frost tender. The soil is still warm, the temperature has mitigated to tolerable and it’s just downright pleasant to be outside. One can consider the seasons somewhat reversed here. Back East, people can hardly wait for winter to be over so they can go out to the nursery or shop on-line for their bulb and Hostas in the spring and get their hands in the dirt. In Phoenix and surrounds, we count the weeks for summer to be over and fall to arrive for exactly the same reason, plus it means our tap water won’t be hot!
And speaking of fall, what causes those leaves to change color and are the different colors specific to certain trees? Autumn leaf color is influenced by three factors: day length, weather and the leaf pigments naturally present. Day length is the biggest and most unvarying factor in leaf color change. Chlorophyll is what give leaves their green color and enables the chemical reaction of photosynthesis, further enabling plants to u e sunlight which is used to manufacture sugars. The e sugar arc then stored for the coming winter dormant period. As the daylight hours shorten, chlorophyll production slows down and then stops. Carotenoids (yellow, orange and brown colors) that are already present in the leaf but hidden by the chlorophyll then show their true colors. The red, purple and crimson tints come from anthocyanin which is produced in the autumn in response to bright light and excess plant sugars in the leaf cells. These colors are also revealed upon the cessation of chlorophyll production.
The yellows and golds (gambell oaks, aspens), because they are constantly present in the leaves, will have a fairly consistent color show year after year. It is the brilliant red, purple and crimsons (maples) that can be affected by weather conditions, mainly temperature and moisture, occurring before and during the time at which the chlorophyll is waning. The most brilliant displays take place when there is a succession of warm, sunny days and cool, crisp but not freezing nights.
Freezing nights … l remember when I was a child, my mom always prayed for a cold winter to kill off all the bugs and black widow spiders. As a plant person, I pray for warm winters so my plants won’t sustain damage. Either way, it is time to consider this season’s frost protection strategies, even though as you read this it is very possibly still 90 or 100+ degrees. Once it really does start cooling off some, ease up on your watering in order to acclimatize your plants. This causes the minerals/ electrolytes in plant tissues to concentrate, effectively lowering the freezing point in the cell and will reduce the stress on your plant if a freeze should occur. Cacti, many succulents and most native trees and shrubs are not actively growing in winter and do not need the soil to be saturated. Most plants are fine with a once per month winter watering if there has been no rain.
If a freeze does occur, use materials other than plastic to cover your plants. A nice product to use N-Sulate, available at most garden centers. This is a lightweight cloth that can lie directly on the plant and allows light and rain to pass through. If rain is associated with a given cold front, be sure any materials used to cover the plants are not too heavy when wet. Materials should be draped to the ground so as to retain any heat the soil and plant may have absorbed during the day, removing covers the next day to let light and heat in. You can also wrap your plants with the old-fashioned holiday lights (incandescent, not LEDs), which will give off a minimal amount of heat. Plastic foam cups can offer additional protection for the stem tips of tender cacti. Although earlier in this article I eschewed the use of plastic, it can be used if draped over a frame to create a greenhouse of sorts. But do not let it touch the plants, as they will burn.
Winter does not use a calendar. One day it will be 80 degrees, the following week the temperature might plunge to 32. That’s what makes gardening so much fun.
Editor’s note: This article by Cathy Babcock was originally published in the October 2012 issue of the quarterly Boyce Thompson Arboretum Member Newsletter. As of this writing (late October), the higher elevations in Arizona are at their peak of autumn color, but the lower deserts, including the famous pistachios at the Arboretum, won’t fully develop their color until close to Thanksgiving. Even so, fall is on everyone’s mind, so it seems apropos to reprise this informative post about what makes autumn tick, and how to prepare your plants for the winter season that is soon to follow.