I don't like the spikes. This type of fertilizer is mostly nitrogen, which burns roots. Let's say you put 5 spikes in around your tree, in an area around each of the spikes it will burn the roots. Then as you get away from the spike about 4 or 5 inches, there will be an area of soil that is getting a dose of nitrogen that may be correct.
So you end up with 5 little circles of burned roots, each with an area around them of about 6" with high nitrogen levels. Then the majority of the root area is unaffected. It would be much better to take 5 nice dog tirds and place them around the tree.
The other obvious problem is water moves fertilizer straight down. so most of the nitrogen just goes into the subsoil and eventually into the water table, killing any roots it encounters as it goes.
What really works nice is the bagged compost they sell at garden centers and department stores. For evergreens, the Composted Sheep Manure might be best because it is slightly acidic. It has a enough nitrogen to promote fast growth, but not enough to burn any roots, even if you put it right on exposed roots. You'll want a 40 lb bag for an area of about 10x10 feet ($2). So 2 or 3 bags per tree.
Then put a decorative mulch down over that, no more than about 2" deep.
Water during drought. Get right down under the tree and poke a finger into the original root ball to see if it needs water. If it's wet don't add more water, if it's dry, water.
Once the roots get established, the top of the tree will take off.
Add more compost and mulch each spring.