Whether it is appropriate to fertilize grass in hot weather depends heavily on the specific type of grass and the fertilizer being used. For most lawns, “hot weather” is defined by sustained daytime air temperatures exceeding 85 to 90 degrees Fahrenheit. Fertilizing under these conditions is generally discouraged because it induces severe stress on the turfgrass. If feeding is necessary, the risk can be managed by understanding the grass’s physiological response and adjusting the application method.
Why High Heat Makes Fertilizing Risky
High temperatures fundamentally alter the internal processes of turfgrass, making it highly susceptible to damage from added nutrients. Cool-season grasses, such as Kentucky bluegrass and fescue, thrive in cooler soil temperatures, typically between 60°F and 75°F. When air temperatures climb above 85°F, these grasses slow metabolic functions and often enter semi-dormancy.
Applying fertilizer to a stressed or dormant plant is ineffective, as the grass lacks the energy and active growth needed to utilize the nutrients. High heat accelerates the plant’s respiration rate while limiting photosynthesis, depleting carbohydrate reserves. This limits the plant’s ability to recover from nutrient application stress.
Fertilizer burn is the primary danger, caused by an osmotic stress reaction. Fertilizers, particularly those high in nitrogen, contain soluble salts. When applied, these salts increase salinity in the soil or on the leaf surface, drawing moisture out of the grass cells through osmosis.
This desiccation effect is magnified because the grass already struggles to retain water due to increased transpiration rates. The combination of heat and high salt concentration leads to the characteristic browning and scorching known as burn. Fertilization also forces new growth, which increases the plant’s overall water demand, potentially leading to severe drought stress.
Understanding Fertilizer Types and Their Role in Heat
The type of nitrogen source in the fertilizer determines the level of risk when applying it during periods of elevated heat. Nitrogen fertilizers are broadly categorized by how quickly their nutrient content becomes available to the plant. Quick-release fertilizers, based on water-soluble nitrogen (WSN), pose the highest risk of fertilizer burn and should be avoided entirely during hot weather.
These fast-acting formulas release a large concentration of nitrogen and associated salts into the soil almost immediately after being watered in. This rapid, high concentration of salts causes the severe osmotic shock that burns the grass. Quick-release products are designed for rapid green-up during periods of active growth, which is typically not the case during the peak of summer heat.
If summer feeding is necessary, the only viable option is to use a slow-release fertilizer, which contains water-insoluble nitrogen (WIN). These products release nutrients gradually over an extended period, often through polymer coatings or microbial breakdown. This controlled delivery maintains a lower salt concentration in the soil, reducing the chance of osmotic burn and preventing the sudden flush of growth.
Examples of safer summer fertilizers include products with coated urea, urea-formaldehyde, or natural organic fertilizers. Organic fertilizers rely on soil microbial activity to break down and release nutrients, and this process accelerates naturally in warmer soil temperatures. This gradual release makes them a reliable choice for providing sustained nourishment without overwhelming the stressed turf.
Essential Application Adjustments for Summer Feeding
Successfully applying fertilizer in hot weather requires precise modifications to the standard application protocol to minimize stress on the turf. The timing of the application is a major factor in mitigating heat stress. Fertilizers should be applied during the coolest part of the day, specifically in the very early morning or late evening. Applying product during the peak heat of the day maximizes the risk of burn.
A crucial adjustment is to reduce the application rate significantly below the manufacturer’s recommended dosage. Applying a lighter, or “half-rate,” feeding ensures that the grass receives a minimal amount of nutrients without the high salt load. This lighter application provides a maintenance dose rather than an aggressive growth stimulant.
Proper watering is the single most important step when fertilizing in the heat. Water the lawn briefly before application to hydrate the grass and cool the leaf surface. Immediately after the dry granular fertilizer is spread, thorough watering is necessary. This immediate irrigation washes the fertilizer particles off the grass blades and moves the salts into the soil profile, where they are less likely to cause contact burn.
Another simple practice that reduces overall stress is raising the mowing height. Taller grass blades provide more shade to the soil, helping to keep the root zone cooler and reducing moisture loss.