I haven’t tried upsidedown planters but if I do, I’d add water gel crystals to the soil. Just one teaspoonful, when hydrated, holds over a quart of water and releases water to the plants as needed. I use them in hanging baskets to cut down on watering. I’ve used them for 3 years and originally got mine from some firm in San Antonio TX.
That's an interesting idea. I'm in the process of building a "square foot garden". The inventor of these recommends creating a soil mixture using 1/3 vermiculite. Vermiculite is expensive and hard to find in large quantities. As far as I can tell the purpose of the vermiculite is just to hold water, and obviously the gel crystals work incredibly well for that.
"...I’d add water gel crystals to the soil. Just one teaspoonful, when hydrated, holds over a quart of water and releases water to the plants as needed..."I have a giant pumpkin plant that is growing like crazy. Last time I measured, it was growing by over a foot a week at its' main vine. It demands watering EVERY DAY, and I'm afraid it will die when we go out of town this weekend.
I'll definitetly investigate your water gel crystals.