|115 Degrees Fahrenheit. In the shade. On the East side of the house. At 5:00 PM CST|
One could say that is is just a little warm today here in the Mojave Desert on June 21st. This is day three of temperatures over 112 for us here in Sunset Zone 9a in Fabulous Las Vegas. We are on the East side of town but low enough at the base of the foothills that it's not the coolest part of town. That would be up on the slopes of Mount Charleston, where it seldom exceeds 95. But there are warmer spots such as down in Laughlin, NV where it got up to 120 yesterday.
So, what does a gardener do when it's this hot? Try to keep perennials as comfortable as you can. That means three things: mulch, shade and water. Mulch we've already taken care of as mentioned in previous posts. We've supplemented it a bit with the stems of 'bonus plants' which have either ended their annual life cycle already and/or succumbed to the heat.
|Dead 'bonus plants', dormant Bermuda grass (also bonus) and wood chips insulate the soil here.|
In the case of this West facing car port, all we can do really is try to insulate the creeping acacia with mulch and try to keep a few standing 'bonus plants'.
|Tree and shrub branches under cloths keep these shelves shady.|
We've also amped up the shade with trimmed limbs and cloth on the shelves along the East wall. As well as moving more shelves over there to help get more of the plants into afternoon shade. The big elm tree helps (and the tree itself is doing well thanks to the soaker hoses) shade the plants for part of the afternoon, but unfortunately, the setting sun gets under it in late afternoon.
Our new catclaw acacia will someday be able to handle the heat fine on its own, but it's still a little sapling, so we are trying to help it out with extra afternoon shade......
|This old lawn chair is just right for PM shade for our little catclaw acacia.|
......courtesy of an old lawn chair that I keep around even though I probably should get rid of it. I'm glad I kept it this year as it's fairly tightly woven mesh back is just right to give this little catclaw acacia some afternoon shade when the heat is the worst.
Then, there is water. It's tricky to deal with in this heat as you don't want to over-water anything and drown the roots. On the other hand, plants evapotranspirate (a combination of purely physics based evaporation along with plant energy requiring transport of water from roots to the rest of the plant) and use water to cool themselves more than what they actually use to grow. This is especially the case when it is so hot outside. It's one thing for a plant to keep its cells cool while they grow, respire and (in some cases) carry out photosynthesis - all of which produce heat in the plant. It's quite another when the air outside the plant is even warmer than the plant's normal temperature.
When that happens, most plants will take up more water than usual as long as it isn't too hot. When it gets above 80, most plant processes begin to slow down and growth usually stops. Almost all plant processes stop when it gets well above 100. Then the plant enters a sort of dormancy state but it is not as dormant as some plants get when it is too cold in the winter and still needs water and other nutrients. Then, when it cools off at night (even as hot as its been here, it gets down to the high 70's - low 80's at night this time of year), the plant will resume growth and most other processes. I've observed the most growth on closely monitored plants in the morning after a cool (or at least somewhat cooler) night.
Even 'drought tolerant' and desert native plants behave this way to a greater or lesser degree. It's part of what makes them able to live in such hot places as they are better at shutting down and insulating their stems and leaves from outside heat than plants from wetter, cooler places are. So, we increase our watering times from the once a day schedule in late Spring (the weather, never what the calendar says) to two times a day (early morning and early evening - never water in the middle of the night as that's a ticket to fungal problems) in June to ever six hours or so during the day when it is above 110.
We can do this without over-watering the plants because we are using soaker hoses which put out about 1/4 a gallon of water a minute for every 10 feet of hose under the typical house's hose bib delivery capacity of 30 gallons per minute. And we run the hoses for 30 minutes each time. That comes to a total of 3 gallons per foot of hose covered area each day. This is enough, when you estimate losses from evaporation, to get about an inch of water in the top 2 inches of the soil (if the soil drains well) and hopefully, part of an inch soaked down deeper into the soil to encourage a deeper, better root system. Evaporation is not a total loss though, because it helps cool the soil, plants and the air immediately around the plants. Yes, it is good to try to minimize evaporation, which is part of what mulch and shade do, but some evaporation is necessary, especially when it is this hot.
|This clay pot without a drain makes a great evaporation basin.|
That's why I encourage evaporation pans like these around plants in dry climates. It helps simulate an environment that you'd have near a stream or pool here in the desert where the more showy plants that we like to have live in nature. This clay pot in the above picture doesn't have a drain hole. I found it at a thrift store and figured it was a perfect little mini-water storage basin. Placed under the soaker hose, it catches some of the water and stores it, releasing it by evaporation slowly thanks to how small diameter it is relative to how deep it is. Plants nearby enjoy a somewhat more humid, cooler microclimate like what they'd have at the edge of a pond.
|This volunteer sunflower wilts each afternoon.|
For some plants, all of this effort isn't quite enough, but some are hanging in there and kinda bouncing back by morning like this volunteer sunflower. This sunflower is getting some water from a soaker hose, so it does well in the morning and early afternoon, but is rather wilty by evening. If it survives to bloom, great. If not, I won't be terribly surprised.
|These wildflowers are doing just fine.|
Other, more desert adapted or native plants are doing much better like these wildflowers from a mix that I broadcast last fall. In fact, they didn't bloom until a couple of days ago.
We'll see how everything turns out once things start cooling down again. I wish you luck with keeping your plants happy (or at least alive) this summer!