When to Fertilize
When leaves, needles and twigs decompose on the ground, they provide nutrients naturally to plants. Often this is adequate fertilizer, especially for native trees and shrubs.
Store-bought fertilizers contain nitrogen (N) and other ingredients. When a plant gets extra nitrogen, it will grow more rapidly, which can result in better flowering, denser leaves, and larger plants. If you have sandy soils, adding phosphorous (P) will benefit your plants. Colorado soils contain plenty of potassium (K), so it is good to buy fertilizer that has as little as possible. A soil test will reveal what nutrients exist on your property.
Our alkaline soils can have little iron and other micro-nutrients available for plants. Iron chlorosis, where the leaves of a plant - turn yellow or whitish, it is a common sign of low iron levels. If considering a fertilizer with iron in it, look for chelated iron, as it is much more bio-available to plants than the less-expensive iron sulfate.
Organic fertilizers have become more popular and more available, but are more expensive and their nutrients release more slowly than chemical fertilizers. Be sure to avoid fertilizers that contain salt, like steer manure, as many Colorado soils have high salt content.
Often when we see a plant struggling, we think fertilizer will make it better, but often the opposite is true. Fertilizer can burn stressed plants, or signal for them to grow when they are struggling. Also avoid using fertilizers on new plantings; let them adapt and grow some before fertilizing
Fertilizer is best used on plants at their peak growing cycle: just before flowering or leafing out. With plenty of moisture, plants can continue to take up fertilizer during their growing season, but discontinue use by fall.
Plants need more fertilizer when they get full sun and plenty of water. Conversely, plants in low-light and those in dry areas need much less fertilizer. Over-use of fertilizer can result in “leggy” or spindly plants, or can run-off into streams, causing environmental problems.
For more information, see the following Colorado State University reference materials:
- Plant Talk 1608 – Fertilizers
- Plant Talk 1619 – Organic versus Manufactured Fertilizers 1708
- Garden Notes: 232, 234
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Updated Thursday, February 18, 2016