|My mother-in-law has a barrel cactus that grew about four inches in a 2-3 month period. It is no longer short and round but tall and cone shaped. The new growth is very light in color. What could have caused this?|
|Your cactus does sound somewhat confused! If it's a true barrel cactus (Echinocactus grusonii) it should have retained its barrel shape and gotten larger in circumference, not taller. I wonder if your plant isn't really Haageocereus choisicensis,a similar cactus that starts out with a barrel shape but eventually becomes columnar.
The light green color is typical of new plant material, so don't be too concerned - it will deepen in color as it matures. You didn't, by any chance, feed it heavily? That would account for the uncharacteristic amount of growth.
At this point, just make sure the plant is in as sunny a spot as you can find, and provide very little water during the winter - just enough to keep the plant from shriveling.
Watch for formation of a flower - that's the real indication that the plant is happy in its location and with the attention you're giving it.