Tips for how to fertilize wisely
Q: How would I know which plants in my yard would benefit from routine fertilizing next year? There are so many fertilizer options in stores that it’s hard to know where to start.
A: In general, the garden plants that might benefit from routine fertilization would be turfgrass and edible crops (fruits and vegetables). We ask a lot of our lawns and harvestable plants in terms of vigor and productivity, so the boost from added nutrients can help keep them at peak performance. Container-grown plants, particularly annuals that bloom all summer, will deplete their nutrient supply over time and do benefit from regular in-season fertilization. Otherwise, most trees, shrubs, vines, perennials, bulbs, and several annuals grow and bloom just fine without fertilizer. Plus, any tree remotely near a lawn will have plenty of roots infiltrating the area to pick up unused nutrients.
We tend to think of fertilizer as plant “food,” but that’s not quite accurate. It’s more akin to a multivitamin, helping to support healthy growth if the plant is growing in the conditions it needs to thrive. It won’t be an elixir for plants ailing due to pest or disease outbreaks, or from conditions that stress roots or stunt growth. Therefore, don’t turn to fertilizer if a plant isn’t thriving unless factors other than nutrient deficiency have been ruled out.
The good news is that, when fertilizer is warranted, the wide array of formulations available can help you customize, to some extent, what to apply so you minimize adding unnecessary nutrients. How do you tell what’s needed? Get a soil test.
It will measure specific nutrient levels plus soil pH, as this impacts nutrient availability. Some labs can do added assessments for elements like lead, but they don’t screen for plant diseases or for pesticide contaminants. Somewhat akin to a routine blood test for people, which have ranges of results deemed “normal,” you’re looking for deficiencies of key nutrients rather than targeting a specific value as a goal.
For lawn care, a soil test is the only way to know if Maryland’s fertilizer law permits the use of products containing phosphorus. By avoiding over-applying certain nutrients, we help protect surface water and groundwater from contamination.
We have a page dedicated to soil testing and interpreting the results, titled “Soil Testing and Soil Testing Labs.” You can also read our “Fertilizer” page (plus others specific to certain plant groups) to learn more about the functions of specific nutrients, the ways fertilizers are applied, and to see an overview of some organic product options. Finally, you can reduce fertilizer use by increasing your soil’s organic matter content.
Q: When should I repot houseplants?
A: Spring is the oft-recommended time, but it can be done any time of year. How urgently the plant needs to be given more root space or fresh soil is the deciding factor. If not urgent, you can wait until spring because the increasing hours and intensity of daylight usually spur growth, allowing roots to establish faster in their new home.
If you need to correct for poor soil conditions — too “salty” with fertilizer or hard water residues, or too poorly drained — then repot as soon as possible. The goal is to minimize root stress, so waiting until spring may only make matters worse in that case. When the soil needs renewing, remove what old soil you can before filling-in with fresh.
Newly-acquired plants should be given at least a week or two of acclimation time before shifting them to a new pot. They have to contend with a change in light, temperature, and humidity levels, which may require the shedding and regrowth of leaves as they adjust. Root disturbance during this period may create extra stress, so try to give them one hurdle at a time.