Last fall, I planted a bunch of fruit trees around my house in Fair Oaks, CA. While I was shopping at the nursery, I eyes widened when I saw they had guava trees for sale. They were growing in a green house though because guava is a tropical plant. We have an amazing climate here for growing plants (USDA zone 9b) but we do get some frost every winter which is kind of a problem.
One of the most common solutions people do a lot around here, mostly for young citrus trees that haven’t been established yet, is to cover the plants with a bedsheet overnight when a freeze is forecasted. This is a lot of work though because you have to take the sheet off during the day or the plant with suffocate so if you’re out of town, then your plants might die.
So I was looking into more automated solutions to this problem. Lots of people use incandescent Christmas lights to which give off warmth, but most people seemed to recommend doing this in addition to the bed sheet to hold the warmth in. Without the bedsheet, I would worry that the warmth is too localized and the extremities will still have frost damage.
I searched online about what California citrus farmers do to protect from frost damage, because there’s no way they cover their trees with bedsheets. I wanted to discover a more scalable solution. It turns out, they just turn their sprinklers on. Because we don’t have a hard freeze here, maybe 28°F at the coldest, then misting water in the air won’t immediately freeze and will instead raise the air temperature to above freezing. So that’s the strategy I went with.
Of course, I want it to be entirely automated, so I used an Arduino with a temperature sensor and a relay to control a sprinkler valve, which is connected to a drip zone that has some misters.
The hardware was quite simple (and cheap).
Setting up the Arduino was surprisingly easy. On mac, it’s just brew install arduino-ide.