Extreme Heat and the Effect on Grass

Extreme Heat and the Effect on Grass

We all possess a collective understanding of how to respond when faced with a heat warning: prioritize hydration, apply sunscreen, and seek indoor refuge if possible. However, amidst the scorching heat warnings that have spanned across Canada in May and early June, an often overlooked concern emerges—our lawns. Many of you have expressed inquiries regarding lawn stress, dormancy, and the safety of employing liquid fertilizers. Allow us to address all of these queries and dispel any uncertainties surrounding the matter.

What is cool season grass?

Cool-season turf refers to grass species that thrive in regions with cooler climates, such as fescue, ryegrass, and Kentucky bluegrass. These grasses have a natural growth pattern that is adapted to cooler temperatures and can become stressed when exposed to high heat.

Why does cool season grass go dormant in high heat?

Temperature stress

Cool-season grasses have an optimal temperature range for growth, typically between 60°F and 75°F (15°C to 24°C). When temperatures rise above this range, the grass can experience heat stress. High heat can cause damage to the plant's cells, leading to wilting, browning, and even death. Dormancy allows the grass to conserve energy and protect itself from the extreme heat.

Water stress

Heat often accompanies dry conditions, and cool-season turf requires an adequate water supply to survive. During hot periods, water evaporates more quickly from the soil, and the grass may not receive enough moisture to support its growth. Dormancy helps the grass conserve water by reducing its transpiration rate (water loss through the leaves) and slowing down its metabolic processes.

Lack of root development

Cool-season grasses have shallower root systems compared to warm-season grasses like Bermuda grass or St. Augustine grass. The shallow roots make cool-season turf more susceptible to heat stress since they cannot reach deeper soil layers for water and nutrients. When the soil temperature rises above what the grass can tolerate, the roots struggle to function efficiently, and the grass enters dormancy as a protective measure.

Energy conservation

Going dormant allows cool-season grasses to redirect their energy and resources towards essential functions, such as root maintenance and survival. By reducing above-ground growth, the grass can allocate more resources to root growth and development. This helps strengthen the grass's ability to recover once the heat stress subsides and cooler conditions return.

"My grass is dead, isn't it?"

NO! It's important to note that although cool-season turf may go dormant during high heat periods, it is typically a temporary state. Once temperatures cool down and adequate moisture becomes available, the grass can recover and resume active growth.

8 Best Practices While in a High Heat Period of Summer


Water deeply and infrequently to encourage deep root growth. Watering deeply encourages the roots to grow deeper into the soil, making the grass more resilient to heat stress. Watering early in the morning or late in the evening reduces water loss through evaporation. Typically we want to provide around 1 inch (2.5 cm) of water per week during the summer months, including rainfall, however during a extreme high heat period evaporation can hinder this and while you may think you have watered enough it probably isn't enough. We recommend to push that during a high heat period to 2 inches or even more!

Adjust mowing height

Raise the mowing height to reduce stress on the grass. Longer grass blades provide shade to the soil, helping to retain moisture and prevent weed growth. Set your mower to the highest recommended height for your specific grass species. Avoid cutting more than one-third of the grass height in a single mowing.

Mow in the evening

If possible, mow the lawn in the evening when temperatures are cooler. Mowing during high heat can add stress to the grass and increase water loss.

Aerate the soil

Core aeration helps improve soil drainage and reduces compaction. By creating small holes in the soil, aeration allows better air circulation and water penetration, promoting healthier root growth. It also helps the grass access nutrients and reduces the risk of disease.

Monitor for pests and diseases

High heat and humidity can create favorable conditions for pests and diseases to thrive. Keep a close eye on your lawn for signs of insect damage or disease symptoms, such as discoloration, thinning, or unusual patches.

Avoid foot traffic

During stressful periods, minimize foot traffic on the lawn as much as possible. Extra stress from constant foot traffic can weaken the grass and inhibit its ability to recover.

Choose Heat Resistant Seed Cultivars

The majority of Kentucky Bluegrass cultivars, commonly found in sod and general seed blends, have a tendency to go dormant at lower temperatures then elite varieties. To mitigate heat stress and minimize damage to your lawn, it is essential to select elite seed blends such as SR2100, Mercury, or Fielder during the seeding process. These high-quality blends have been bred for specific characteristics such as improved heat tolerance, drought resistance, reduced mowing heights, disease resistance, and leaf colour and texture. This will help ensure the health and vitality of your lawn.

Be patient

Remember that cool-season grasses naturally go dormant during high heat periods as a survival mechanism. With proper care and patience, the grass will usually recover once cooler temperatures return and conditions become more favorable.

Its way too hot to fertilize, right?

Applying liquid foliar fertilizers to grass during a period of high heat, when done properly, does not lead to the immediate death of the grass. While it is important to exercise caution and consider the specific conditions, here are a few reasons why properly applied foliar fertilizers will not kill the grass during high heat

Quick nutrient absorption: Liquid foliar fertilizers are designed to be quickly absorbed by the leaves of the grass. This allows the grass to uptake essential nutrients directly, bypassing the root system. Even during high heat, the nutrients can be efficiently absorbed by the grass, providing it with essential elements for growth and health.

Nutrient supplementation: During high heat periods, the grass may experience increased stress and have limited access to nutrients from the soil. Applying a properly balanced foliar fertilizer can help supplement the grass with essential nutrients, which can support its overall health and resilience during stressful conditions. This supplementation can help the grass maintain its vigor and recover more effectively once the high heat subsides. Remember that we are spoon feeding the grass, so not applying all the fertilizer at once like some granular products do.

Dilution and proper application: Liquid foliar fertilizers are typically diluted with water before application. Diluting the fertilizer properly ensures that the concentration of nutrients is appropriate for the grass. Additionally, following the recommended application rates and guidelines provided by the manufacturer is crucial. When applied correctly, the risk of damaging the grass is minimized.

Improved nutrient availability: Liquid foliar fertilizers can enhance the availability of nutrients to the grass by providing them in a form that is easily absorbed. This can be especially beneficial during high heat, when the grass may have reduced nutrient uptake due to heat stress. The quick absorption of nutrients through the leaves can help ensure that the grass receives the required elements for its growth and metabolic processes.

While liquid foliar fertilizers, when properly applied, are generally safe for grass during high heat, it is crucial to consider the specific conditions and the grass's health. If the grass is already severely stressed or experiencing other issues, it may be best to delay fertilizer application until more favorable conditions return. Additionally, ensuring proper watering and other stress management practices will further support the grass's ability to withstand high heat periods.

So what does this all mean for Growforge Products?

The application of Growforge products remains safe even under high heat conditions, provided that proper application techniques are followed. Optimal results can be achieved by applying the products in the evening when the sun's intensity has diminished, adhering to the recommended application rate without exceeding it. Additionally, it is crucial to ensure adequate watering for your lawn throughout the week. The inclusion of wetting agents like TPA will improve moisture retention in the soil and can assist during these high heat periods.

As always, if you have any further questions, please send them to info@growforge.ca. We will reply to them as soon as we can!

Stay cool!

The Growforge Team