By Jagathi Kamalakanthan, NCDA&CS Agronomic Services Div.
Spring is the best time to fertilize your shrubs as it promotes optimal growth with healthy foliage and abundant flowers. Well-fertilized shrubs not only improve the aesthetics of the landscape and provide wildlife habitat, but also enhance value and appearance of the property.
The NC Department of Agriculture and Consumer Services’ Agronomic Services Division encourages home gardeners, community gardeners and landscapers to submit soil samples to optimize growing conditions. From April through mid-November, soil testing is free, and samples are typically turned around within two weeks.
“Fertilizers provide nutrients to help plants grow, but it is important to apply the correct amount and type of fertilizer to promote optimal plant growth,” said Jagathi Kamalakanthan, an NCDA&CS soil testing agronomist. “If the soil pH does not match the plant needs, then fertilizing alone may not be helpful.”
It is also important to follow the lime recommendation on the soil test report, Kamalakanthan said. For most shrubs, a pH of 6.0 is optimal; however, acid-loving plants such as azaleas, camellias, rhododendron and mountain laurel prefer a pH of 5.0.
While a lack of proper soil nutrients can lead to poor growth, other environmental factors may be an underlying contributor. Those include inadequate sunlight, soil compaction, poor drainage, insects or diseases.
Before deciding fertilizer is the answer, determine why plants are growing poorly, then correct the factors contributing to poor growth, Kamalakanthan said. Fertilization may help, but only after other problems are corrected.
“Whether you are working with new or established plants, fertilization should be based on soil test results,” Kamalakanthan said. “Applying too much lime and fertilizer, or routinely applying unneeded fertilizer, can be a wasteful investment and pollute the environment.” Also, it can hinder the plant’s ability to take up nutrients or create toxicity, depending on the nutrient.
Many urban soils are low in phosphorus, which can present challenges with new plantings. Phosphorus helps promote growth and the establishment of plants. “Having the soil tested before putting in new plantings can help ensure a better start, if phosphorous and other nutrients are needed,” Kamalakanthan said. Phosphorous also works best if mixed or incorporated into the soil rooting zone.
For new plantings, wait until spring to fertilize fall-planted trees and shrubs. Wait six to eight weeks to fertilize plants installed in the spring. Fertilize established trees and shrubs in the spring as new growth begins. From an environmental perspective, spring is the best time to fertilize since plants are actively growing and taking up nutrients; fall and winter fertilization, especially nitrogen is subject to loss with water through the soil when plants are not as active.
Some general fertilization tips to consider:
- Avoid fertilization in late summer (around mid-August) since it may stimulate late-season growth that fails to harden off before frost;
- Do not use slow-release fertilizers after July 15;
- Apply fertilizer evenly around the plant root zone, keeping it away from the bark and foliage;
- Plan to do a soil test every two to three years;
- Plants typically need fertilization every year but not lime. Follow the lime recommendation on the soil test report and apply lime only as recommended by soil testing.
A detailed pictorial guide to collecting and sampling for home gardens is available at www.ncagr.gov/agronomi/pdffiles/HomeApr2014.pdf.
More information on fertilizing trees and shrubs can be found at content.ces.ncsu.edu/a-gardeners-guide-to-fertilizing-trees-and-shrubs.
Contact the Agronomic Services Division office at 919.733.2655 for questions regarding soil testing.