June 29, 2021
We’ve received quite a few questions about best practices for fertilizer applications during hot weather, so here is a set of guidelines for midsummer and hot weather fertility management in lawns, flowers, trees and shrubs, vegetables and fruit.
- 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.
- Avoid washing granular fertilizer into hard surfaces like sidewalks, driveways, and streets.
- 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.
Withhold lawn fertilizer applications until moisture returns and lawns are actively growing. The next-best time to fertilize could realistically be this fall.
Right now unirrigated lawns are in a drought-induced dormancy.
When making fertilizer decisions, consult our guide to fertilizing lawns and our Minnesota lawn care calendar.
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.
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 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. Use Extension’s diagnostic tool What’s wrong with my plant? and Ask a Master Gardener for assistance.
Trees and shrubs
Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens and Landscape Plants
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 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 (yes, you already have to think about winter!)
Ideally, most vegetable crops should be fertilized in the early spring at planting time. You might notice that your plants are looking stunted right now, but this is more likely a result of the heat and drought than a lack of fertility.
There are a few cases in which it might be worth adding fertilizer at this point in the season:
Each fruit crop has its own separate guidelines for when and how to fertilize. Refer to the University of Minnesota Extension Yard and Garden fruit webpages to find fertilizer recommendations for your fruit crops.
With the exception of strawberries and raspberries, most of the fertilizer application for other fruit crops is focused on the beginning and end of the season. Mid-season fertilizing is mainly used to correct nutrient deficiencies, which are indicated by foliar nutrient testing and visual symptoms.
If you suspect that your plants may have nutrient deficiencies, first make sure you are watering sufficiently. Water stress can impact nutrient uptake. You can submit soil samples to the UMN Soil Testing Laboratory.
Phosphorus and potassium fertilizers do not move easily through the soil. Therefore, sprinkling granular (solid) N-P-K fertilizer pellets on the soil surface mid-season is not likely to incorporate P and K as intended. It is best to mix those nutrients into the soil before planting or use water-soluble fertilizers during the season.
This press release was produced by the University of Minnesota Extension. The views expressed here are the author’s own.