Im assuming by Echinocactus Grusonii you mean the golden ball cactus. First off I think you could wait until the spring to repot it. I would probably just water it every month to keep it healthy and let it go dormant. Is there a place where you could keep it around 55-65 degrees? Also mix the commercial cactus mix, perlite and the orchid fines in a 1-1-1 ratio. It should be very well drained but if not add a little perlite until its good. As for taking it out of the pot wrap a strip of corrugated cardboard around the ball and use that to hold onto it. Use that also to put it in its new pot. I must also express that these can get 3 feet tall and wide

Top heavy equals very dry. Unless the cactus looks seriously thirsty right now, I'd keep it that way until you repot. If you can manage to get the peat wet, you'll never get it dry again! Keep it as cool as possible all winter but preferably above freezing. Too warm and it will grow, which won't be a pretty sight in winter.

Come spring, repot as already described. What you pot it in isn't too important but it must drain well and keep any peat to a small proportion of the mix.

Summer outdoors is great. Once it is hot and sunny it will take a lot of water so the occasional summer thunderstorm isn't a problem, but make sure excess water can escape freely through the holes in the bottom of the pot. Watch out for it scorching in spring as it gets used to the sun, and watch out for signs of stress on hot afternoons. Shade the pot to keep the roots cool, and shade the whole plant if it still gets stressed.

I will share a piece of advice given to me by Patrick Quirk, who was the curator of cactus and succulents at Desert Botanical Garden for many years. This man was a living encyclopedia of plants and has forgotten more than I will ever know.

"When transplanting cactus plants from pot to pot or from pot to ground, consider the soil types you are dealing with. If the new soil is different than the soil the plant was in previously, it is best to bare root the plant (knock all the old soil off) and let it rest for a week or so before replanting in the new soil. If the soil mix is the same, the plant can be transplanted directly from the old soil to the new."

This advice has served me well. Also, I will recommend using a mix of commercial cactus mix and perlite in a 1:1 ratio (do not use the orchid bark or peat). I say this because this particular plant does not need much in the way of organic material in the mix, and there is a little of that in the cactus mix already.

One more thing: when you place the plant outside next spring, somehow mark the plant or container showing which side is facing south. Keep that side of the plant facing south all through the growing season, and the following year when you bring it back outside, make sure to orient the south side of the plant south again.

Thanks all for your help! It is definitely an Echinocactus grusonii - the name on its tag just says barrel cactus, but it has those golden spines and yellow fuzzy top.

I must also express that these can get 3 feet tall and wide

I know :-). There is one that size in the C&S greenhouses at the Montreal Botanical Gardens. It's unlikely this one will get that big though - partly because I don't have a greenhouse for it, and partly because I am such a cactus newbie that I am unlikely to be that successful!

So, I will plan on repotting it in the spring. Since it's in a mostly peat based soil right now I'll bare root it for a week as suggested. For now I will just place the plactic pot inside a terracotta pot to give it a bit more stability (it is quite tippy). It doesn't look like it is thirsty right now - it it very firm to the touch in the couple of areas I can touch it.

One more thing: when you place the plant outside next spring, somehow mark the plant or container showing which side is facing south. Keep that side of the plant facing south all through the growing season, and the following year when you bring it back outside, make sure to orient the south side of the plant south again.

I will do that, but I am curious as to why it isn't a good idea to turn the plant. Won't this encourage uneven growth?

BP

I am curious as to why it isn't a good idea to turn the plant.

The idea is not to scorch the plant by exposing skin that isn't acclimatised to direct sun. You are going to be dealing with this issue every spring and I don't think which way the plant is facing makes so much difference in our northern climates. See what you find, much better to damage a five dollar plant that you can replace than to grow something into a show winning specimen and then suddenly get a burn mark that will never go away, just because you turned the pot!

The plant won't grow distorted if it is kept outside, even if it is never turned. But on a window sill, one side will definitely grow differently and you'll have to turn it.