Answer: You'll want to train your tree while it is still small so begin by looking at the tree to see whether it has more branches on one side than the other. (This may account for it listing to one side.) If it's overly heavy on one side, remove some of the branches so it won't pull in one direction. Once you've evened up the number of branches so they're equal on all sides, stake the tree so that the trunk will grow upright. It's okay to have branches that cascade down, but you'll want the trunk to be upright so it can support all of the branches and leaves. The best way to stake a tree is to use 3 stakes spaced an equal distance from each other and tie the trunk to each individual stake, exerting more pressure on the tie that will hold it most upright. Periodically check the ties to make sure they are not cutting into the bark and tissue of the trunk. The stakes can probably be removed in 2-3 years and your aspen should be strong enough to hold itself upright without stakes.
Best wishes with your landscape!
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