How Much Fertilizer is Too Much for Thuja, Arborvitae, Cedar, Cypress?

When plants are over fertilized the plants will appear to be drying out the same as if they were lacking water. In essence the plant will not be able to absorb water and the surrounding environment will absorb the water from the plant. Also, with over fertilizing by the time we made a mistake, it is probably too late to fix it.

Nurseries grow trees for sale, so we apply the maximum nutrients and water the plants will tolerate.

Plants in pots are in a confined space, over fertilizing will definitely kill these plants. Plants established outside in the ground are much harder to kill by over fertilizing as the roots reach far and it is unlikely that the entire area is over fertilized.

In a 7 Gallon pot for Thuja and Cypress a small fistfull of 10-10-10 fertilizer is about the maximum safe amount. In a 15 Gallon pot about a single fistfull is the most we would apply. Plants already established can take a couple of fistfull around the base or the tree.

A good way to gauge the nutrient need of the tree is looking at its color: if it is not a “mean” or deep green, it probably needs balanced macronutrients. If it is full of new growth, it is probably healthy with plenty of nutrition.

Apart from macronutrients, trees thrive on the micronutrients of leaf or other organic matter as well. This is especially true for Thujas.