We receive lots of questions at OSU Extension about mid season care of your garden and landscape plants. Here is a quick set of guidelines for midsummer and hot weather fertility management in lawns, flowers, trees and shrubs, vegetables.
General tips
Avoid applying fertilizers when plants are dormant or under heat and drought stress. Fertilizers should be incorporated into the soil to prevent losses. If you cannot physically incorporate a fertilizer at this point in the season, it may be best to stick with a water-soluble fertilizer.
Fertilizers are not accessible to plants in very dry soils. Water the soil before applying fertilizer, allowing water to soak into the ground. Water again immediately after fertilizing. Liquid fertilizer is a good option if a plant is showing nutrient stress and needs nutrients right away. Slow-release fertilizers dissolve with watering and release nutrients in small doses over time.
A soil test every 3 to 5 years will help you understand the health of your soil.
Lawns
Withhold lawn fertilizer applications until moisture returns and lawns are actively growing. The next-best time to fertilize could realistically be this fall. Typically, Grass growth will remain slow during July and August due to high temperatures.
Ornamental flowers, evergreens, trees and shrubs
When you see active growth on a plant (new leaves and stems, buds and flowers, increased size), that is a good visual cue to fertilize.
Annual flowers
Mix in a granular, general-purpose fertilizer with equal amounts of nitrogen, phosphorus and potassium into the potting mix
at time of planting (if the mix does not already contain a fertilizer). Annual flowers usually bloom heavily, so fertilize every 2-4 weeks with a water-soluble fertilizer.
Perennial flowers
Perennial flowers benefit from well-composted manure worked gently into the root zone of your garden plants in early spring before perennials get too large. If you notice a perennial flower looking off-color or hardly blooming, try to determine if the plant is under stress (heat and drought is prevalent this year) or if it is lacking some nutrients like nitrogen, iron, magnesium, etc.
Trees and shrubs
Newly planted trees and shrubs benefit from slow-release and natural organic fertilizers incorporated into the backfill soil. Young trees benefit from fertilization early in the year. Fertilize established (mature) trees and shrubs in late fall or early spring when active growth occurs.
Evergreens
Spring is the optimal time to fertilize evergreens as new growth starts to appear, and you can fertilize up until mid-July. We recommend stopping all fertilization after mid-July because it can prompt new growth on the plant that may not harden off sufficiently before cold weather sets in. The only exception: if your evergreen is nutrient-stressed, apply a slow-release fertilizer to help the plant move into winter and keep watering evergreens regularly into late fall.
Vegetables
Container-grown vegetables require more frequent applications. Ideally, most vegetable crops should be fertilized in the early spring at planting time. In container-grown vegetables, nutrients tend to leach more readily, and so adding a little bit of fertilizer to your water each week can help container-grown plants to stay healthy. But a few vegetables like peppers and cucurbits (pumpkins, zucchini) are very sensitive to excess nitrogen, and may significantly delay their flowering if they receive too much nitrogen this late in the season.
Apply about half of the fertilizer your tomatoes will require as fruit begins to appear. If you already applied enough at the beginning of the season, do not apply more now.
Source: University of Minnesota Yard and Garden Line