Planting in Summer?

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We are told repeatedly to plant either in the fall or early spring in desert areas (I myself encourage this in my classes and lectures). Summer planting is to be avoided like the plague. But what do we do if we end up getting a plant just before summer, or worse yet, in the middle of summer? Should we plant that potted specimen during a heat wave, or keep it in its pot until fall when the air is cooler, the soil is cooler, and the sun is less intense? This can be a gut-wrenching decision for gardeners who want to do the right thing, but worry about subjecting their new plant to either the ravages of the extreme heat and intense sun if planted in the ground in summer, or to the poor prospects of a plant surviving a desert summer in a black pot with hot roots and wet soil. Hmmm…which is harder on a young plant: hot soil in the ground or hot soil in a pot?

I have had this discussion with many of my gardening friends who are experts…those who are professional nursery growers themselves. They all wince when they think about planting in summer in the desert, but they tend to agree that of the two bad choices, the best option is to go ahead and put the plant into the ground, installing a special protective shield for the plant during summer plantings.

We are advised to not plant in hot summer months for many reasons: plants require more water in their first and subsequent years when planted in summer instead of fall or spring, they may succumb to root rot if the soil is kept moist when the soil is very hot, and plants may grow slower than if they had been planted in fall or spring. However, there is a real danger of the plant not surviving the summer at all in a pot. All plants, but especially southwest native plants, hate for their roots to be hot if the soil is wet. Our native southwest plants are adapted to go dormant in summer when soils are dry, so their roots are used to being dry when it is hot. In a pot however, especially a black pot, the soil and roots heat up even more than if the plant were in the ground, and to compensate, we may have to water the plant almost daily to keep the roots from overheating and drying out beyond their tolerance. However, extremely hot, wet soils are a breeding ground for fungus and bacteria, which can proliferate in these conditions and overwhelm the roots to the point of root rot and sudden death.

Plant roots that are buried in the ground, even when exposed to the same air temperature as a potted plant nearby, are living in a soil environment that is not nearly as hot as the plant sitting next to it, but above-ground, in a pot. One way to lessen the impacts of this “black pot syndrome” is to put the black-potted plant into a bigger ceramic or non-black pot to shield the roots from the heat of the direct sun. This definitely helps, but I have still had some potted plants succumb to summer heat as I waited until fall to plant them. Our fall planting window in southern California usually this doesn’t arrive until the first of November–which is a long wait if you acquired a plant in summer.

If you have a potted plant you have adopted just as summer sets in, here is a suggestion for planting during the heat to give it the best chance of success: immediately after planting, install a shade barrier on the south (sunny) side of the plant using three stakes in a “V” shape, either made of wood, metal, or PVC pipe sections. Use stakes tall enough to cast a shade onto the entire plant. Onto these three stakes, attach shade cloth with either twist ties or a stapler, so the plant receives shade most of the day as the sun moves across it. Install the “V” middle-point as close as possible to the plant to give it as much shade as possible. Check your shade screen throughout the day to check its positioning, and pull the sides in closer if needed to cast shade onto your new planting. Do not put shade cloth over the plant, as the cloth will trap heat under it and heat up the plant even more. A large rock placed on the sunny (south) side of the plant also helps your plant to stay happier. The rock will shade the ground next to the roots, and retain moisture longer under it. The companion rock also helps to shade the plant, and can even encourage some dew to form and drip off the rock as cool night air condenses on the rock still warm from the day’s sun.

Figuring out how often to water a newly installed plant during summer is a delicate situation as well–too wet, and it rots; too dry, and it suffers. The root ball needs to be kept moist, but wet soil at the surface where soils are very hot can cause excessive growth of bacteria and harmful fungi, resulting in root rot and killing your plant. The soil needs to be kept moist about 3 inches below the surface, but keeping the soil continuously wet at the surface can be detrimental.  Checking soil moisture is helpful here: either use an inexpensive moisture meter (less that $9 at gardening stores), or you can use an old screwdriver to dig a small hole to a depth of about 2-3”. Check for moisture at the bottom of the hole. After watering, if no moisture has reached the bottom of your test hole, keep watering. Use a new hole to recheck moisture with your meter or hole after watering again. Allow the soil in the top 2-3 inches to dry out between watering the newly installed plant.

I’m writing this article now because several months ago I decided to wait until fall to plant a very special native plant I have wanted for years–Yerba Santa, which a fellow botanist had gifted to me. It has been thriving for three months in the pot…that is until we had a 118 degree day in the midst of weeks of over 100 degree weather with extreme humidity. The Yerba Santa has taken a nose dive, even though I have had the pot itself shaded. I lament now not planting it even though summer was setting in when I received it, since I fear I have now lost the treasured plant. As a rule of thumb, plants that cannot tolerate wet soil during summer (which include some of our southwest native plants), should not be planted in summer even if the above measures are followed. Examples of these plants that hate summer water and need dry soils during summer include Ceanothus (California Lilac), Fremontodendron (Flannelbush),  Arctostaphylos (Manzanita), and Trichostema (Wooly Blue Curls). For this reason, the easiest plants to get established in the summer are those that can tolerate year-round moisture.

I’ll never know for certain if my Yerba Santa would have survived the heat in the ground during this excessive heat wave, but I know for sure it hated being in the pot as summer put its burning fangs around my tender plant. Hopefully these tips can help you can avoid this sad experience.

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