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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.
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.
- 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.
Lawns
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.
- Grass growth will remain slow during July and August due to high temperatures.
- Do not apply large fertilizer loads (1 lb N/1000 sq ft) when lawns are dormant or slow-growing. Grass will not take up the nutrients applied, and those nutrients are wasted and could pollute the environment.
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.
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. Use Extension’s diagnostic tool What’s wrong with my plant? and Ask a Master Gardener for assistance.
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. Note that these trees and shrubs do not require a lot of nitrogen and typically get enough from lawn fertilizers applied nearby.
Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens and Landscape Plants
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 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!)
Water Wisely
Vegetables
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:
- 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. Read more about fertilizing plants grown in containers.
- If you’re planting a new set of vegetables, such as midsummer broccoli, green beans, or carrots that you plan to harvest in the fall, you’ll want to apply more fertilizer (this can include synthetic or organic fertilizers). Make sure to water it well since plants cannot use nutrients in dry soil efficiently.
- If you have very sandy soil, apply half or a third of your fertilizer in the spring at planting time, and then add the rest over the next month or two. This works well for many crops grown in sandy soil. 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.
Fruit
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.
- June-bearing strawberries should be fertilized immediately after the end of the harvest season, usually early July. This happens during the renovation process, which every strawberry grower should do every year.
- Day-neutral strawberries may need 2-3 light sprinklings of nitrogen fertilizer throughout the harvest season, in order to help them keep producing large fruits during the very long season. Be sure to water thoroughly after applying nitrogen, so that it enters the soil where the roots can reach it.
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.
Authors: Natalie Hoidal, Annie Klodd,Maggie Reiter and Julie Weisenhorn, Extension educators
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