Generally, fertilizers should be applied just before the plants begin growing vigorously, which is when they need the most nutrients. This usually means the time when trees and shrubs leaf out in the spring. Plants use mineral nutrients to make new leaves and to form flower buds, but not to open their flowers. Follow the old adage to “feed the leaves, not the flowers.”
To be of the highest quality, vegetables need to grow as rapidly as possible. For this reason, they should have more fertilizer more frequently than other plants. Prepare the soil with lots of compost and fertilizer to give them a good start. Then side-dress when the plants are 4 inches high and again as they begin bearing fruit or reaching harvest size. If the older leaves begin to yellow at any point, feed them again.
You can also use slow-release plant food, applied when the plants are set out. One feeding of a slow-release food will last all season.
The exception to this rule is tomatoes, which will stay in their immature state and grow leaves at the expense of flowers and fruit if you give them too much nitrogen. Hold off on nitrogen until the first fruit is set. Once they begin bearing, they will go on setting fruit no matter how much nitrogen you give them.
Spring-flowering Trees and Shrubs (Including Fruit Trees)
As with other trees, feed spring-flowering trees when they begin to leaf out in the spring, not when the flowers open. These trees set the buds that will become next year’s flowers (and fruit) in mid-summer. Be sure they have plenty of fertilizer then to get a good flower and fruit set for next spring.
At planting time, add a high-phosphate fertilizer to the bottom of the planting hole. Feed spring-flowering bulbs nitrogen when leaves are reaching full size. At this point, they begin growing the bulbs for next year’s bloom.
Feed cool-season lawns 4 times: when they begin growth in the spring (with moderate amounts of nitrogen), in early summer (lightly), when the nights turn cool in the fall (heavily), and as soon as top growth stops in late fall (lightly). The roots go on growing for a couple of weeks after top growth starts. This last feeding allows the grass plants to store nitrogen in their roots for next spring’s growth, allowing a quick spring start-up.
Feed warm-season grass when they begin growing in the spring, then every 6 weeks until they begin to go dormant.
Stop fertilizing trees and shrubs in mid-summer. Late fertilizing encourages growth that would be too young and tender to withstand winter stress.