Vermicomposting in Arizona Summer Heat?

A great question from Marie:

I used to do worm composting when I lived in Washington state. Now that I have moved to Arizona, and it is approaching summer, I am concerned about temperatures too hot for worms. I don’t have space to keep a bin inside, so that isn’t an option. I was thinking of using a container garden that I built using large concrete blocks. (The rectangular kind with two big holes in them that are used for walls). It is 2 feet x 6 feet wide, and 2 feet deep. It is shaded during quite a bit of the day by a large citrus tree. It gets regular water through a drip irrigation system. I was going to use a thick layer of pine needles to cover the top and encourage the worms to move deep into the center of the bin. Do you think that this would work?
Thank you for your expertise!

Hi Marie,
You are 100% correct in assuming that outdoor vermicomposting during an Arizona summer will likely be very challenging. But the good news is that the idea you shared sounds very much on target in terms of the sort of system that could potentially work!

For the benefit of others reading, I am going to start by laying some groundwork based on what we are dealing with here. To be honest, I don’t really know how hot it can get during an Arizona summer, but I’m am going to guess that 100+ F is pretty common over the course of several months.

Short of setting up some sort of power-sucking, artificial cooling system, typical free-standing enclosed plastic “worm bins” are just not going to work. Wooden systems, and other breathable systems (eg Worm Inn) might be possible under the right circumstances. Lots of shade. Lots of air flow. Lots of moisture.

The one nice thing about Arizona is the low humidity – this makes it easier to employ evaporative cooling strategies.

Just so you know, the worms can likely survive at temps up to about 94 F – but keep in mind that temps inside a vermicomposting system can often be even warmer than ambient temps.

OK let’s chat about your idea…

What I love about the idea is that it is close to the ground, open, sheltered (in terms of location and materials used), and irrigated. That sounds like a magic combination to me!

That said, there are likely still a couple of tweaks I would make:

1) Instead of pine needles, I would likely use straw/hay or something like layers of burlap (latter might work really well for evaporative cooling). The needles will probably be a pain to work with and they will increase acidity and maybe add resins to the bed that aren’t all that worm-friendly.

2) Something to think about before you set anything up is creating a decent pit down below. The earth itself serves as an amazing buffer against extreme temperatures (both hot and cold). If you dig down a couple of feet and add lots of moistened cardboard (brown corrugated cardboard is probably my favorite – and I think the worms agree), this will provide the worms with a place they can retreat to if conditions in the main bed end up getting too hot.

But like I said, you are absolutely on the right track!

Hope this helps – and please do keep me posted on your progress.

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    • Walt Pendleton
    • May 11, 2018

    I have lived in Arizona for many years. I set up a in ground worm bin 4 x 8 foot. with concrete blocks. I put a wire hardware cloth to keep varmints out. I got shredded newspaper and covered it with old rugs.
    I have extra worms if anyone in Arizona wants them.

    • don
    • May 11, 2018

    Hi Walt, Where in Arizona are you? Do you sell castings?

    • Bentley
    • May 11, 2018

    That’s great, Walt (and great to hear from you)
    Thanks for chiming in!

    • Walt Pendleton
    • May 12, 2018

    I am in Surprise Arizona. I do have worm castings as well as worms.

    • Walt Pendleton
    • May 13, 2018

    Give me a call if you want free worms or castings. 623-734-5097

    • John W.
    • May 13, 2018

    You can also set up a Sprinkler system. I had a mister that turned on for 15 min once an hour. It was long enough that top layer got wet but short enough that the worm bin did not get overly wet. You would have to play with the setting with the system you might get. I liked the mister (kind of like the ones they have at them parks that just keep people from passing out), because water in my area is not cheap. Just bought a digital timer that let me adjust when and how long and I was done.

    • Marie
    • May 14, 2018

    Thank you all for the tips. I will use a different mulch and add a couple extra misters to the drip system.

    • clare
    • June 28, 2018

    I want to set up some worm towers in my garden in spain, 45Km inland from Malaga. Do you know where I can bbuy worms which will withstand the heat annd can you make suggestions as to how to go about it.
    thank you

    • Bentley
    • July 11, 2018

    Hi Clare,
    In ground systems like worm towers are likely your best bet for beating the heat, so that was a good idea. Unfortunately, there are no composting worms that are especially well-adapted for really hot conditions, but assuming your towers are deep enough in the ground, I would think you could get Red Worms to survive ok. Maybe keep a small indoor “insurance bin” just to be on the safe side. Here is an article I wrote on the topic:

    • Shaul
    • August 17, 2018

    Solar-reflective, Mylar plastic sheeting works great at cutting temperatures. Position it over your bins wherever the Sun will hit and if you can have it not touching the bin, so that there’s room for airflow, then even better.

    • Caleb
    • January 4, 2019

    @Shaul – I LOVE that idea, I never thought about that! Mylar sheeting is used all the time in indoor grow tents to keep the light reflecting in the tent, but I didn’t think of it for reflecting the outside of the worm bin! I’m totally going to line the outside of my WF360s with mylar sheeting now great idea!

    • Shaul
    • January 4, 2019

    Caleb; I used to work in a large warehouse for a company that supplied pharmaceuticals and beauty products to pharmacies. Many times, pallets of cartons containing heat-sensitive products, would arrive covered with Solar-reflective Mylar plastic sheeting. Since the company considered it a waste product, I took it home. Now it’s being repurposed keeping wormbins cool in summer and warm in winter. Try to keep a few inches between the Mylar and whatever you’re covering to allow for airflow.

    • Bentley
    • January 8, 2019

    Fascinating topic guys!
    Shaul – I would love to see a comparison of temp readings (with and without the mylar). Maybe set up dummy bins (or at least one for the full exposure treatment) just to make sure no worms get cooked in the process.

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