Shrub Fertilization

The right fertilizer at the right time


Shrubs will benefit from an annual application of fertilizer in spring or early summer (March through June). Slow release (WIN - Water Insoluble Nitrogen) or organic nitrogen is a better choice than quick release nitrogen since it feeds a shrub more gradually and reduces the chance of fertilizer "burn."


Don't fertilize shrubs in the midwest or northeastern US after July 4th since it may encourage a late "flush" of new growth that might not get "hardened off" before winter cold. It's best to apply granular fertilizers around the base of a shrub without putting any in contact with the foliage or the trunk.

Liquid Fertilizer

Liquid fertilizer is different than granular products, with label recommendations often calling for applying it over the foliage for additional uptake of nutrients through the leaves. Early morning is a better time than the heat of the day for these sorts of applications.

Always read and follow fertilizer label instructions. More fertilizer is not necessarily better since many fertilizers have a high "salt index" which increases their "burn" potential on plants. It is always a good idea to water fertilizer into the soil following an application. If you use wood mulch regularly it is a good idea to use nitrogen fertilizer annually to help offset what the breakdown of wood mulch robs from the soil.

Evergreen shrubs

Evergreens will benefit from the use of a product like 'Miracid', especially if they are showing any signs of chlorosis (yellowing between the darker green veins of the leaf). Chlorosis is often caused by a shortage of iron or other micronutrients Miracid contains. Miracid gets mixed with water according to the label directions and sprinkled over the root zone and foliage, since some of the fertilizer is absorbed through the leaves.

A cool morning is better than a hot afternoon for this sort of foliar application. An application of Miracid once a month in April, May and June should be adequate for shrubs with chlorosis. These liquid feedings can be supplemented with a single application of an organic fertilizer like Milorganite during that same time period.

Rhododendrons benefit from liquid fertilizer applications containing trace elements.

Rhododendrons benefit from liquid fertilizer applications containing trace elements.

Since most granular chemical fertilizers are "salty" it is always best to water them into the soil.  To further eliminate the chances of any fertilizer burn, don't fertilize plants that are "bone dry" -- water them first before fertilizer is applied. Same applies to houseplants and potted plants. Be sure to keep granular chemical fertilizers away from trunks and off of wet foliage. Lightly work fertilizer into the top layer of soil if you can do so without damaging surface roots, but avoid this step on shallow-rooted plants (if in doubt do not cultivate fertilizer in). I would also recommend applying fertilizer prior to mulching if you are doing both.