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I brought into the home all of my potted Cactus. They all over-wintered in my living room. It is now March 12, and even though the
temps here are still cold and Winter is still here, when do I resume watering and at what temps can I place them all back outside?
They are all desert cacti, with alot of Echinopsis, species and hybrids.

Once its consistently in the 20's (C)to be safe.

I’d wait.

i have been keeping cacti for many years in heated rooms over the winter and outside in an open shelter in summer.
I water very lightly about once a month in winter to prevent root loss. in March I up the frequency to every 2 weeks, but still a small quantity( not enough to drip). When I see growth I water throughly, but still every other week.
I move my plants outside "when danger of frost is passed" I rush this date some, knowing I may have to protect the plants from frost.I water carefully to avoid having the plant's roots wet during cold cloudy stretches of weather that are common here in spring.

Echinopsis are pretty cold-hardy. So with them I'd be happy to go for "when danger of frost is passed".

Our Last Frost Date is March 31st. I'm going to have to watch the temps and weather to see if it is appropriate to put all of my cactus outside.

A slight frost (say, up to -3C) should not even be a worry to most of your plants on a regular basis. A bit colder is also not a big thing, if that does not happen too often. Just make sure they are still dry. If you place them outside, I would advice you to shield them with a transparant covering. Then they can enjoy all the sun they get, but are still safe from excessive rain.

All my plants are back in the greenhouse as of this weekend. But there is still a chance of light frost up here until May. But no expectations of -3 or colder.