I have planted 2 plum trees about a month ago and I am wondering when should I fertilize them?
Most of the info I have found tells me to do this the first year. Do they mean now or after the first year?
Fertilization during the first year is appropriate, but wait until you see some new growth in the tips of the foliage, indicating that the tree has recovered from the planting process and begun to put out new absorbing roots. Be conservative and use a slow release natural/organic fertilizer product. Harsh chemical fertilizer can burn and cause dieback. Mulch the soil surface to help retain soil moisture and enhance soil organic matter and benefical biological activity.
Here's an article that may help:
"As a rule, avoid fertilizing new plants until they establish. It can cause a burst of growth that makes the plant weak and leggy."