Increasing overnight temperatures spell out bad news for plant growth. How do we help?

July 2023 was a record breaker for the warmest month ever recorded on Earth, and records indicate overnight lows in Kansas City have increased by 2 degrees. The most noticeable impact of overnight temperature increases is on plant growth. Each plant has a specific temperature range in which it will grow best. Even small changes of a degree or two can adversely influence plant growth.

Let’s take a closer look at this relationship.

When nighttime temperatures are higher, plants have an increased respiration rate, meaning they consume more energy. Nighttime should be a period of rest for a plant. Instead, the plant is forced to focus more on respiration, uses more energy than it generates and produces less energy during daytime photosynthesis. Less energy means the plant has fewer food reserves for growth, flowering and fruiting.

Researchers at Kansas State found that a 1-degree increase in overnight temperatures may decrease wheat yields by as much as 5%. Our garden vegetables, fruits and flowers also suffer similar decreases in growth and yield production.

Decreased growth rates affect the plants in several ways. Plants require more water as they transpire to cool themselves. Water usage increases as the plant consumes more during increased respiration. Warmer temperatures also significantly increase evaporation from the soil. This means it requires even more water to keep the plant properly hydrated. Lack of water, like high temperatures, limits plant growth and development.

Higher temperatures also affect plant pollination. Nighttime temperatures above 75 degrees cause pollen grains, which should be sticky and moist, to dry out. This prevents the successful transfer of viable pollen from the stamen to the ovaries. Poor fruit set in tomatoes is an example of this poor pollination. The result is a reduction or lack of flower and fruit production.

Additionally, temperature increases affect the roots and change the dynamics of what is below the ground. While warmer soil temperatures increase nutrient availability, they also greatly influence microbes. This may sound good, but it ultimately results in more losses throughout the soil profile.

Microbial activity is vital for a healthy plant, and the connection between compromised roots, reduced nutrient uptake, and decreased microbial activity to increased soil temps cannot be underestimated. All these factors combined make the plants less efficient in taking up water and nutrients to promote growth.

Another fallout from increased temperatures relates to color and quantity.

The intensity and quality of flower colors are impacted by higher overnight temperatures. Warm temperatures result in less vibrant colors. Pigments that normally develop rich color in flowers do not adequately develop. Cooler overnight lows result in richer, purer shades of reds and blues. Remember, when plants expend more energy overnight on respiration, they also produce fewer flowers.

As you can see, the increased nighttime temperatures have an overall negative effect on plant growth.

What can a gardener do? Good question.

It goes back to the basics of right plant, right place. Good cultural practices in the garden are also crucial. But it is so much more, and many of these changes are out of our hands. Understanding these changes helps us plan for the future of gardening in Kansas City.

Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Need help? Contact the Johnson County Extension gardening hotline at 913-715-7050 or email garden.help@jocogov.org.