Just called manufacturer. The heaters are designed to work in room that is 68 degrees>. That leaves my garage out where the temps still get down in high 20s or 30s.

Good call calling the manufacturer.

Submersible heaters would seem to need liquid.

I do not like adding liquid, electricity, breakable glass all together.

Some ideas that may be of interest are plant growing mats or brewing belts designed to keep a volume of liquid warmer. Reptile mats if waterproof.

Those hand warmer things do not seem to last until their expiration date. So those for extra cold nights will not work.

You're probably right I think I'll forget about submersible heater. Tried heating mats last yr don't think it worked very well. Something simple that I overlooked is warming the water up before I put in bucket.

Hate to throw a curve into this, but if you're brewing tea (aerated vermicompost tea?), the necessity of a heater will guarantee that the biology in the tea will meet an icy death when removed from that heat source and distributed into the cold, hard facts of unlivable conditions for that biology.

AVCT needs to be spread in warm, tropical-like temps. 60 above is good. 70-80 ideal.

The little critters are delivered to the intended locations wet, naked, and a little fatigued by all that helter-skelter lifestyle you've put them through.

Chuckiebtoo

OK then. Looks like I will settle for a simple fert program til June unless el nino throws a better curve.