Considering summer fertilizer for your garden? Proceed carefully, or risk doing more harm than good.
For some plants, midsummer fertilizing can provide a needed boost after three months or more of vigorous growing. But for many others, fertilizer can be unnecessary or even harmful.
“Only fertilize plants if you’re sure they need it,” said Julie Janoski, manager of the Plant Clinic at The Morton Arboretum in Lisle. “Applying fertilizer when it isn’t needed or at the wrong time, or applying too much, can dry out plants’ roots or create chemical imbalances.
“If you over-fertilize plants, you may get too many leaves at the expense of flowers,” she said. Surplus fertilizer also finds its way into rivers and lakes and causes serious pollution problems.
Trees and shrubs rarely need fertilizer. “We don’t usually recommend fertilizing them unless a soil test has shown a specific nutrient deficiency,” she said.
Most perennials don’t need fertilizer either, as long as they are planted in soil that has adequate organic matter. Organisms in healthy soil will digest the organic matter to provide the nutrients the perennials need.
Which plants do need fertilizing? “It’s usually plants such as flowering annuals that are growing in unnatural circumstances, such as containers, where they can’t access nutrients from a wide area of soil,” she said. “In those circumstances, they quickly use up the available nutrients, and they can use some extra help.” A fertilizer boost can also help vegetables keep producing through the harvest season.
Fertilizer is not plant food. Plants make their own food through a chemical process called photosynthesis that captures energy from sunlight. Fertilizer can supplement plants’ food with an increase in certain chemical elements they need to build large molecules.
In nature, plants get all the elements they need from the soil. In our home landscapes, which are often very different from plants’ natural homes, certain plants may develop deficiencies that fertilizer can supply. For example, if a pin oak tree is planted in alkaline soil rather than in the acidic soil this tree species prefers, it may develop an iron deficiency that can be helped by fertilizer. A bur oak tree, which is adapted for alkaline soil, would not need it.
Here are some guidelines from Janoski for summer fertilizing:
Use slow-release. Fertilizer formulas that dissolve slowly into the soil will release nutrients gradually and avoid overdosing plants. Organic fertilizers are all slow-release, as are some synthetic formulas.
Check the ratio. The label of every fertilizer product has a ratio, such as 3-1-1, indicating the percentages it contains of nitrogen, phosphorus and potassium, the elements that plants use in the largest quantities. “Look for numbers in the low single digits, which indicate that the product is a slow-release formula,” Janoski said.
More is not better. Never exceed the amounts recommended in the directions. “Err on the side of using less,” she said. “If you’re using a liquid fertilizer that is dissolved in water, you can play it safe by using half the amount recommended on the label.”
Water first. The chemicals in fertilizer are basically salts and can easily dry out plant tissue. For insurance, make sure the plants are well supplied with water before you fertilize.
Apply at the right time. Different plants may need supplemental nutrients at different times. Fertilizing in summer can help annual flowers keep blooming until frost and keep vegetables producing. Lawns, though, are best fertilized in early fall, when they have begun growing again in cooler weather, and the fertilizer can help them grow strong roots. In the rare instances that trees or shrubs need fertilizer, it is best applied in spring or fall. Ask the Plant Clinic for advice on whether and when to fertilize particular plants.
Fertilizer is not a cure-all. “The only thing fertilizer can help with is a shortage of particular nutrients,” Janoski said. “If that is not the cause of the problem, fertilizer will be useless.” Always identify the reason that a plant is not growing well before you decide whether fertilizer might be useful.
Test your soil. “A soil test can tell you the chemical composition of your soil, to help you choose plants that are likely to grow well without the need for fertilizer,” she said. Learn more at mortonarb.org/soil-test.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
