Too much fertilizer may harm your plants

My mother just returned from a garden tour of England, and her normally green thumb is two shades greener with envy. With the milder and wetter climate there, plants seem much bigger and lusher.

It’s tempting to achieve that magazine cover look by giving our trees and shrubs a little extra fertilizer. If a little is good, a little more must be better, right? Wrong! While annual flowers and vegetables may require it, trees and shrubs do better without too much fertilizer. In fact, excess applications do much more harm than good.

Adding fertilizer at the wrong rate can burn root tissue, or invite insects and disease to attack the softer growth it promotes. Fertilizing at the wrong time can push new growth when trees should be hardening off for winter; this new growth freezes and dies, opening routes for decay.

You’ve probably also heard about high phosphate levels threatening our rivers and aquifer. Fertilizers contain phosphates, so putting too much into our landscapes can easily pollute our water through runoff and leaching.

Answer these questions before adding fertilizer. Are your plants already getting nutrients through composted mulches and organic matter in the soil? Have you had your soil tested and did the results showed deficiencies? Are your plants showing smaller than normal leaves and twigs, off-color foliage or dead twigs and branches? Are these symptoms caused by an insect or disease?

If you do need to fertilize, make sure you apply the right amount by following the label. Choose one that adds nutrients that your soil test shows are missing. If you don’t do a soil test, stick to a fertilizer with a nutrient ratio of 3-1-2 or 3-1-1. Slow release formulas are generally better for trees and shrubs. Water after every application. Fertilize from late winter through spring. Stop after mid-July.

Signs of overfertilization include a stunted plant, foliage that is unusually large, or shoots that are much longer than normal. Aphids favor overfertilized plants, as do some fungal diseases such as powdery mildew. If you suspect you’ve given your woody plants too much, you can help by giving them extra water, and looking out for signs of insects or disease that may require treatment.

Contact a certified arborist, a knowledgeable nursery or the Master Gardener Plant Clinic for more information about fertilizing trees and shrubs.