I realize this is a very old post, but figured I'd say...it's a species of Senecio.

Thanks for all the advice everyone, I appreciate it! I think it might be a Euphorbia after looking at some photos online, too. I guess I'll find out when I cut the tops off, which, by the way, thank you for the warning about the latex (juice?). First thing I did when I got it today was water it since it hadn't been watered in months. I now realize this was probably a mistake. I may be able to wrestle it out of the pot tomorrow to check the roots and re-pot dry in a fast-draining potting mix. What signs of rot/healthiness should I be looking for? Any tips for how to take this out and re-pot it? Can't say I've ever wrestled with a 5 foot spiny plant before and some of the main arms are weak/soft. Speaking of, should I cut off any smaller arms that are black/collapsed? And can I do anything about the mold spots? Or does it all depend on the condition of the roots? P.S. This is the photo that was used to advertise the plant, taken probably 2 months ago before the tenants of the place moved out and left it in an unheated basement. In Michigan (during winter).

I think it should be "Mammillaria matudae". It depends more on the size of the root ball than the size of the plant, but it sounds as if it is time to repot it. The pot should not be too big, perhaps an inch wider than the root ball. A clay pot could be wider as it dries out quicker, I believe.

Be assured that there are no "dumb" questions except those not asked. The way we learn is by asking questions. A good place to start any lawn care project is with a good reliable soil test for soil pH and major nutrient values and balance and that can be had from your local Louisiana State University Cooperative Extension Service office. Also these simple soil tests, 1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top. 2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsâ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up. 3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart. 4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell. 5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy. may be of some help. Once you know more about the soil you have then you can begin to plan on how3 to make it into a good healthy soil that will grow a strong and healthy lawn. Here is a link that might be useful: LSU CES