For years I kept my overnight low in the greenhouse at 40-45 degrees on the coldest nights and was very happy with the performance of my taro, cannas, umbrella palm, papyrus, rushes, thalia, etc. I must say though, because of the lower temps and the lower intensity of winter sunlight, I experienced a great deal of rot when the plants are left in water. Since I began dry potting my aquatic marginal plants in compost or sterile potting soil with good drainage and water on all sunny days (evaporation in the pots is less on the cooler cloudy days in Oklahoma), my losses are now usually related to something stupid that I’ve done.
In regard to tropical water lilies, the ambient temperature being low doesn’t seem to be as much a problem as does the water temperature. Though it may not be cost effective I drop 800 watt submersible titanium heaters in tubs or vats to elevate the water temp around 60 or 65 for mature tropical lilies and 75-85 if I’m trying to stimulate young growth from tubers.
By the way, when my marginals go back in the water in the spring, I remove the winter potting mix and transplant back into my trusty Oklahoma red clay with a thin layer of Osmocote 13-13-13 slow release fertilizer under it. Water gardening is the only good use I ever found for that darned clay.
I know most people will be shocked at the fact that I take a lot of hardy aquatic plants in for the winter but my Koi are too large and numerous to have much success dropping them in the bottom of those ponds, plus I’ve learned to like the idea of starting the season each year with larger root stock on my plants.
Enjoy outgrowin’ yer greenhouse.
Paul.
