Hey Anne!
I don’t really have a tried and true way to do this, but this is what I do. I start with fresh potting soil. That means your window boxes need to be free of any old soil from the summer before. If you do have old soil – try and clean out the top layer of soil if you can, this will help hold your spruce tips in place.

Once you get your tips in your box, and your boxes are looking pretty – give the old box a good watering. This will allow the dirt to freeze, and your tips should be frozen in place in a few short hours. That doesn’t mean they won’t fall over, you’ll have to keep a close eye on them for 1-2 hours after you arrange your boxes while the tips are freezing in place.

Hope that helps, good luck!