Non-grafted Astros are among the more cold tolerant cactus species around, most can easily survive frosts if dry. Non-grafted A. asterias and the kabutos are by far the most finicky, having a tendency to rot for no apparent reason especially as the plants get larger. Make sure there is lots of drainage material in the mix, and don't use peat at all.

In my opinion all Astros (and other cactus for that matter) should receive a dormancy period where temperatures are cool and the plants kept dry, light is not important but fresh air is. Warm winters are unnatural and if the plant is stimulated to grow then it is almost impossible to provide enough light for good compact growth. Many cactus require the cold to initiate flower buds. About 6-8 weeks of cold, dry conditions are probably enough for plant health, but it is easier in the north to keep them dormant for longer. I keep mine asleep from mid-November to late-February.

You need to find out what the grafting stock is and follow their needs. Some pereskiopsis and other similar stock are not as cold tolerant, and may need higher winter temps, something like say 45F minimum. They probably also need more watering and maybe less light. The reason for grafting some kabutos is because they are weak on their own roots and have very low photosynthetic activity; they need the stock to stay alive.

In terms of watering non-grafted cactus in the winter, it depends on your soil mix. I use 50% garden soil which is heavy in clay so I don't water during the cold winter but I know the soil does not really become bone dry, there is just enough moisture to prevent root desiccation. No matter what, you should reduce watering drastically. Depending on your soil, an occasional very light watering or misting might be necessary to prevent root loss.

Definitely get a good room fan and keep it circulating 24-7.

Thanks for your response. We are in the DC area here.

I think the graft stocks are Justibertii and hylocereus (spellings)? We posted a link above that shows the astros we have. What is the best way to get them down to 55¼ in an apartment? We do have about a 12 feet long windowsill that may get colder during the evening. Usually we keep the heat about 68 - 70¼ during winter. Do you think its possible to do a light cycle proceedure by lowering hours of light somehow?

If you like we could share our e-mail or phone number or we could ring at a good time for you on our dime. We really appreciate your advice! Thanks again.

Click on my user name and you will get to a page where you can click to send me an email privately. I don't use that email address often so just put "cactus" into the message so I know it's not spam.

I don't know how to get cool temps in an apartment. You can play around with opening windows at night but that may not be a practical thing to do long term. When I used to live with my parents, they had a spare bedroom that I used to keep my cactuses in during the winter, and I would leave the window open so that it stayed pretty cold in the room, in the 50's during the day. Needless to say they were not happy about it, and there was always a cold draft from under the door that needed to be blocked, but the system worked. Later on I experimented with keeping them in the basement in the cold and dark, and it worked perfectly. To this day, I overwinter my cactuses in the garage, no light, just cold and dry.

Warm winter days are OK but warm evenings are not. Your plants will tolerate it because cactus are great at dealing with not-so-ideal conditions, but not forever.

Do you have any friends/ relatives with space to spare in an unheated garage or cellar/ basement maybe? You should be OK this winter with the set-up you have, but make sure you keep the plants drier than usual so you minimize any potential growth. With so many plants in your collection you need to think about how you will care for them and keep them healthy for many years to come. You will learn so much over time.

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I'm not too clear on the best winter treatment for these plants on a graft. For Astrophytums on their own roots, a nice cool dry winter is best for a number of reasons. It is hard to see how you can give them that in your apartment.

Even if the windows let in as much wind as mine used to in Falls Church then they will still be too warm nearly all the time. You may be able to keep many of the plants on the balcony (protected from rain?) for much of the winter. All Astrophytums will take temperatures down to freezing without flinching, provided they are fairly dry. They can go down to 20F, maybe even a little lower, for brief periods. If you are prepared to bring them inside during those few really cold snaps that you get each winter, you should be fine. The stocks are a different matter though. Hylocereus, for example, is tropical and any kind of freeze will kill it pretty fast, 50F is considered a safe minimum temperature.

You may just have to keep them inside and watered a little during winter and expect them to keep growing. You may get few or no flowers but then it isn't easy to flower some of the special forms anyway. Do give them maximum light during winter and give them just the minimum water required to keep the stock going and the graft healthy. The stocks you describe can be quite thirsty and will deprive the grafted plant of water if you keep them too dry.