[The following is adapted from the May 1993 issue of The Avant Gardener]. This has long been an accepted but unproven rule. Now a study by Dr. Warren at the North Carolina State University “has shown that root growth decreases as the amount of nitrogen fertilizer is increased. Nitrogen apparently does not enhance regeneration of roots pruned during digging.
"Many studies have demonstrated that after root loss, growth is redistributed in favor of making new roots. Above-ground growth slows as nutrients are transferred to the roots. So it is a mistake to apply fertilizer in an attempt to stimulate top growth, since the plant’s "instinct” is to regenerate a full root system. NCSU experiments showed that replacing damaged roots is slowed at a rate directly correlated to the amount of nitrogen fertilizer used. This may negatively impact transplant survival and prolong the establishment period. Little or no nitrogen should be applied in the first year after transplanting.
ECHO asked Dr. Warren to clarify a couple points. Does this apply only when the plant has been dug up and root damage has occurred, or also when a plant is carefully transplanted from a pot into the soil? “I have no direct data, but I believe it applies to all transplanted material since establishment is still dependent upon generating new roots into surrounding soil.”
Do you mean to add no fertilizer or just no nitrogen fertilizer? “The data is only applicable to nitrogen fertilizer. I would make sure there are adequate levels of P and K. Recent information suggests that these two do not interfere with root growth.”
ECHO Staff 1994. Go Easy With Fertilizer After Transplanting Trees and Shrubs.. ECHO Development Notes no. 45