Root starters not really fertilizers but are typically plant growth hormone-based and sometimes with additions of various vitamins (like B1). For the most part, they are really quite unnecessary (especially the B1) for already growing plants but can be helpful when taking cuttings and propagating. They tend to be promoted by nursery staff as add-on sales but without any real understanding of what they can and can't do.

One of the reasons that fall planting of trees and shrubs is so often recommended is that these plants in fall naturally undergo an extended period of root growth and root extension that will continue until the ground gets too cold. Plant roots are quite capable of growing perfectly well on their own without us messing about :-) In more than 30 years of professional gardening and countless 100's of trees planted, I've never used a root starter or stimulator product and never had a tree not do well from just a proper planting technique and adequate water. Just make sure you rough up the rootball or release any circling roots - that is ALL the stimulation or encouragement they need!!

Myth of root stimulators.

Thank you so much!!

Also, when you plant, do you cut some of the roots on trees that when taken out of their pots are root bound? (Cutting an x on the bottom and straight down a few times around the tree?)

I have seen this done in some planting videos, but not in others. I worry any time I have to do something like this. I did not do it on any of my first trees, but I thought I would try it on these last few. What are your thoughts?