Is Worm Castings Fertilizer ever recommended for planting trees, if so how...
Thank you for using Extension AaE and for asking such a great and interesting question!
Worm castings as a fertilizer are usually are not suggested for planting trees in part because of the high variability between different home “batches” or commercial production lots in the essential nutrient elements - nitrogen (N), phosphorus (P) and potassium (K) - also known as the “fertilizer elements”. These ratios and percentages can vary greatly depending upon the composition of the material that was decomposed by the worms; 'you get out what you put in'. So its difficult to make recommendations for application rates that are consistent and reliable.
The following link, while not from a verified science based or university extension source, does have a good general explanation of the concerns around the fertilizer value of worm casings:
Another reason why not, and more specifically with regard to recommended best practices for supplementing nutrients when planting trees and the need to fertilize trees in general, the scientific consensus is that it is just not that important to do so.
Still, if soil conditions are known to be poor planting time might be the only time you do need to consider fertilizing trees. Recent research shows that once a tree is well established, factors like soil PH and fertility are actually of far less concern for tree health maintenance than is the natural balance of adequate moisture and oxygen levels. Mature shade trees should rarely if ever need to be fertilized, only watered in times of drought. Here is a link with some excellent information on this topic: