Good soil, sunshine, water: These are critical ingredients for healthy plants and a beautiful garden. Fertilizer? Not usually, said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle.
“Fertilizer is necessary only in certain specific situations,” she said. “It’s not something you should make a habit of using for all your plants, and it’s not a silver bullet for plant problems.”
Plants in containers do need fertilizer, but most established perennials, shrubs and trees rarely, if ever, need to be fertilized, she said. Even lawns and vegetable gardens can usually get along without it if their soil is enriched with organic matter.
What is fertilizer? Whether it’s an organic or synthetic formula, fertilizer is simply a combined dose of certain chemical elements that plants need. They are often called nutrients.
The major ingredients are nitrogen, phosphorus and potassium. Many fertilizers also contain other elements, such as iron and calcium.
However, good garden soil naturally contains enough of these necessary elements. Organisms that live in the soil break down dead plants and animals, releasing the nutrients they contain, which are then used by plants. In most cases, spreading organic matter such as compost or shredded leaves over the soil to feed those organisms is enough to enrich the soil, making fertilizer unnecessary.
There are a few uses for added fertilizer. Plants in containers need it, because their roots can’t spread out widely to collect nutrients. “They have to live off a tiny amount of soil in a very small space,” Yiesla said.
Annual flowers, which we plant for all-season bloom, can also use the help, and so can vegetables if the soil is not sufficiently rich. “Blooming continuously and making fruit use up a lot of nutrients,” she said.
To supply nutrients at a steady, safe rate, consider using a slow-release fertilizer product, even on the lawn. Never use more than the package label recommends. “An overdose of fertilizer can shock plants and burn their roots, or force them to grow too much foliage at the expense of flowers and fruit,” Yiesla said.
For most trees, shrubs and perennials, use fertilizer only if you have determined the plant is suffering from a deficiency of a particular nutrient. “That’s the only problem fertilizer can solve,” Yiesla said. “Fertilizer won’t fix a plant that is growing in too much shade. It can’t cure a disease, or get rid of an insect, or dry out soil that is too wet. For the majority of plant problems, fertilizer won’t help.”
The Arboretum’s Plant Clinic (mortonarb.org/plantadvice) can help you figure out why your plants are growing poorly and whether fertilizer might be a solution.
But in many cases, not using fertilizer has benefits that extend beyond the plant in question. Using fertilizer unnecessarily contributes to water pollution when the surplus leaches into waterways. It also can harm your plants.
“Fertilizer can push a plant’s growth off-course,” said Julie Janoski, Plant Clinic manager. Fertilizing a newly planted tree, for example, will divert its energy to growing leaves, instead of the more important task of growing roots. If a tree or shrub is already under stress from drought or disease, fertilizing it can make the problem worse.
Some fertilizer products contain other damaging ingredients, too. “Weed-and-feed or weed preventer products include herbicides, which can kill plants that aren’t weeds,” Janoski said. If you spread a weed-and-feed fertilizer on the lawn around a tree, the herbicide may affect the tree’s roots growing beneath the grass.
“Don’t spend your time or money and risk harming your plants by fertilizing without a good reason,” Yiesla said. “Most of the time, plants are taking care of themselves.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (mortonarb.org/plantadvice or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
