[Awar]

I've used these "pound in the ground" stakes made, literally, of fertilizer, with good success ...

Jobe's Fertilizer Spikes for Evergreens: https://www.homedepot.com/p/Jobe-s-4-lb-Evergreen-Tree-Fertilizer-Spikes-15-Pack-01611/202701382

That's exactly what I've been using on my green giants for 3+ years now. My trees are the healthy and growing really fast! Much better than other green giants in our neighborhood.

Use 1 spike per inch of trunk diameter, just inside the drip line as long as it's at least 30 inches away from the trunk.

Tip: pound them in using a rubber mallet after a rainy day.