How Hot Is Too Hot For Plants

Perennial Garden in Summer Heat

Caring For Plants During A Heat Wave

Summer is here and once again it’s delivering a one-two punch of brutal heat waves. Temps are skyrocketing for long periods of time and we humans must remember to keep hydrated and find clever ways to cool down.

But what about our plants?

Plants can’t dive into a nice cool lake or drive over to the mall to enjoy the free AC. They’re stuck dealing with heat and blaring sunlight, like it or not. And while they have some temperature-regulating tools built into their biology, there’s only so much heat they can take.

So, how hot IS too hot for plants?

The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell:

  • Leaves wilt. Water evaporates into the atmosphere more quickly in high heat, draining a plant’s reserves. This is especially true in dry heat.
  •  
  • Flowers, especially young blossoms, drop. Some plants need to forfeit their reproductive goals in order to use their available energy to manage their own cooling systems. Heat waves can mean fewer flowers.
  •  
  • They stop breathing to prevent water loss. Harder to see, but some plants will close their leaf pores (stomata) to keep water from evaporating. This comes at a high price, as the pores must remain open to take up carbon dioxide, which is critical to photosynthesis. Plants with limited CO2 are slower to grow and may attract pests due to a weakened immune system.
Watering Perennials on a Hot Day

What can we gardeners do about it?

Plant the right kind of heat-loving plants for your environment, be it dry or humid. Tropical plants like hot, sticky weather while xeriscaping (drough-tolerant) plants thrive in high-temperature climates that see little rain fall. Both sets of plants are standout choices for enduring heat waves, as they're already adapted to those conditions.

Water and Mulch. Whenever it's over 90, be sure to give your plants plenty of water. This will prevent wilt and give them the freedom to manage their internal systems without worrying about losing too much moisture. By mulching your plants early on (and refreshing the mulch 1-2 times throughout the season) you'll help to keep the surrounding soil cool and limit water loss.

Build healthy soil. Good soils mean more water-holding capacity and less evaporation. Perennial plants are best-positioned to survive extreme temps when their roots run deep and wide and healthy soils make this type of self-care possible.

Plant Defensively. Create some shade, especially from afternoon sun. This may mean planting a large shrub, bush, or structure on the western side of your favorite perennials, or it may mean intentionally revamping a large area, making it a Shade Garden. With the ability to lower temps in your yard by fifteen degrees, you and your plants will appreciate a shady retreat.

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