Tordon question's

Have several nasty Tree of Heaven sprouts that need killing and am going to cut and treat them with Tordon. I've used it a little in the past but it was well into spring. My question's are. Does it need to be a certain temp to apply it and would it be best to wait until the tree's start to leave out? Thought I've read Tordon works year round, but want to make sure so I don't waste my time and product.