How do water retaining gel crystals work ?

This is very interesting, Xela. I don't know the answer, but I do have more questions! I bought some plug plants this spring which were sitting in trays of water retaining gel. Rather than throw it away, I though instead I would dry it out and keep it until I needed some gel in the hanging baskets. I left the trays on a high shelf in my greenhouse, expecting them to dry out pretty quickly. However, they actually seemed to get WETTER! i think they were absorbing moisture from the air, at they just got wetter and wetter until they became completely liquid. They're still sat there. WHAT'S THAT ALL ABOUT???!!!

Yes, the same material is sold as a dehumidifying tool. It does absorb moisture from the air. You can dry it out in the oven, but if it's gone completely liquid it's probably too late.

I don't know the answer to Xela's question. At a guess, the gel for gardeners is slightly different to what Magnus Pike had, so that it does release moisture at the right time.

Oh! Thanks for that, Ukslim. Wish I'd known that before! Thought I was going mad - kept looking for leaks in the greenhouse roof - thought water must be getting into it somehow!

Water storing gels such as Broadleaf P4 are designed to absorb water through run-off, watering or out of the air, but not to release it back to the air. Instead, they store it until the plants require it, releasing the water directly back to the plant as needed via the roots. The roots actively seek out the gel particles and grow right through them, so they have a ready access to the water contained in the gel. I know Broadleaf P4 can give over 90% of the water it stores back to the plant, and can re-hydrate repeatedly over a 5 year period, but don't think cheaper variants have this level of re-useability.