You could try building a false wall with sheets of plastic a few feet back from the windows with the plants in between the windows and plastic. This will create a pocket of slightly warmer space in the cooler apartment. Stick a small ceramic heater in there to bump up the temps to 60 to be safe. Or heating pads often used for rooting and seed starting under the pots can help keep the pots warmer and help prevent rot. You can also take plastic milk and juice bottles, paint them black and fill them with water. Set them between the plants where the sun shines on them and they will absorb heat during the day and release it at night. Covering the floor with sheets of black plastic will bump the temps up a degree or two as well.

lisak1, You have a clever and interesting pond. It is a truism on the forum that every pond is different, which makes sharing fun. Hot water heater! So far so good for you. I too observed the falls kept a nice opening in the ice, but when my attempt failed a bubbler system in the pond and in the skimmer did the job in the coldest weather. I spent some time thinking about my failed winter run. I think the greater the volume of water (and I was running several thousand gallons per hour) and the amount of spray resulting from the flow and the configuration and height of the stream and falls are the key. If I were to try again I would reduce the flow to a low volume and see if my observations were right. As with much of life, dumb luck is a variable, but I got the dumb more than the luck. I posted pictures years ago; maybe you can find them with a search. I might try to find and post a couple here. Good job, keep us informed. Nice pond!

These are my Irises in 1 of 4 old vintage planters I have, all 4 pretty much look the same. They're really happy in these planters!! These planters are right at 3 feet long and pretty narrow. (don't look at my messy planting area)

I usually do 1 organic feeding per year but the majority of my nitrogen comes in the form of urea in the fall. My lawn is about 1/2 acre and too much to irrigate so in the fall I drop urea anytime we will get enough rain to wash it in. Outside of that I use fertilizers from the local farm supply. They have a really good selection and typically I can come up with a combo that supplies what the soil test says it needs.