Want to do a little less work in the garden? Reconsider how much you fertilize, says Todd Jacobson, head of horticulture at The Morton Arboretum. Many plants, including trees, shrubs, and most perennials, will usually be fine without it.
Fertilizer isn’t plant food; plants make their own food through photosynthesis. Fertilizer is a supplement to make sure plants get elements such as nitrogen, phosphorus, potassium, iron, calcium, and magnesium that help them function. Think of it as a vitamin pill, not a meal.
In nature, plants absorb these nutrients from the soil. In farms and gardens, where we often grow plants much more intensively or in conditions for which they are not perfectly matched, we may need to add fertilizer for some plants—but not all.
The Arboretum doesn't fertilize trees and shrubs except in certain situations, Jacobson says. Sometimes new plants will get an application of slow-release fertilizer in their second and third year to help them get established. And in cases where a mature tree or shrub is struggling, and the staff has ruled out other causes such as disease and overly wet or compacted soil, they'll do a soil test to see if there is a nutrient deficiency that might call for an application of slow-release fertilizer in the fall.
"It's the last option," he says.
Giving plants more fertilizer than they need can lead to unnecessary maintenance. Since nitrogen pushes the growth of stems and leaves, fertilizing a hedge can mean you will have to trim it more often, and fertilizing a lawn too much will mean you have to mow it more.
"You're causing more work for yourself," Jacobson says.
Many lawns do without any added fertilizer, but if you want insurance for your grass, apply slow-release lawn fertilizer two to four times between May and November, following package directions, Jacobson says.
A slow-release formula will supply an appropriate quantity of nutrients at a rate at which plants can use them. Applying too much fertilizer at one time can "burn" plants, drying out their leaf tissue.
Using slow-release fertilizers and applying no more than the label says will also reduce the risk of harming the environment. Excess fertilizer runs off lawns in the rain or seeps into groundwater, where its nitrogen and phosphorus cause major water pollution problems in rivers, lakes, and oceans.
The Arboretum only fertilizes the lawns in its most heavily visited areas, Jacobson says. The overwhelming majority of the turf in its 1,700 acres is never fertilized.
The grass does benefit from clippings that are left behind after mowing to decompose. That’s how plants get most of their nutrients in nature—from the slow decay of dead plants and other organic matter on the soil surface. The bacteria, fungi, and other organisms that break this material down release the necessary nutrients and make them available to growing plants’ roots.
Most perennials don't need fertilizing, Jacobson says. The plants that do need it most are annual flowers in beds and those in containers. Since they are blooming all the time, annuals use up nutrients faster than perennials do.
The Arboretum staff uses water-soluble fertilizer for annuals and containers. For the first month, while they are establishing roots, annuals get a dose about every third watering. In the vegetable patch of the Arboretum's Children's Garden, organic fertilizer granules are added to the soil at planting, "and that's it," Jacobson says.
The most critical step in making sure plants get the nutrients they need, Jacobson says, is to regularly top-dress the soil in garden beds with organic matter such as compost.
For tree and plant advice, contact the Arboretum's Plant Clinic.