Effects of Heat on Plants

Heat scorch on elderberry.  Located on the side that faces West.

While our typical summer temperatures are in the 90's, there are those sizzling summer temperatures that reach into the 100's.  When you have consecutive high temperatures, your plants may suffer from heat stress.  So how does How does heat effect your garden?

Heat stress is defined as, "a period in which temperatures are hot enough for a sufficient period of time to cause irreversible damage to plant function or development."  It's not just high day time temperatures that can cause problems plants can also be stressed from high nighttime temperatures and high soil temperatures. Also plants on a west side of a brick wall that retains heat or sun reflected from house windows can cause heat stress damage.

Symptoms of heat stress can be mistaken for pest or disease damage.  So always look at your cultural practices and environmental conditions when deciding what the problem is.  

The damage caused depends on the duration of the heat, age of the plant, the water in the tissues of the plant and a plants ability to adjust to heat.  Wind will add more stress to the plants. Plants in containers or metal raised beds are more vulnerable to heat.

Plants cool themselves through transpiration which is evaporation of water through leaves.  If the roots can't absorb water faster than the leaves lose water through transpiration your first symptom will be wilting leaves.

Moving containers into shaded areas during excessive heat will help prevent heat stress.

Effects of heat on plants:

• Wilting
• Leaf Scorch which is brown dry edges
• Yellowing leaves most notably on the lower part of the plant
• Stunned growth
• Death of a plant
• Curling leaves are a response to heat and water loss
• Sunburn on flowers and fruit
• Leaf drop. The plant is trying to minimize the amount of water loss through transpiration common on trees.
• Higher risk of pests and disease when plants are heat stressed
• High temperatures above 100 can cause tomatoes to stop the ripening process or flowers to drop

Leaf scorch on Bishop's Weed.  A great living mulch for shady areas.

Ways to help landscape plants deal with heat stress

So there are some preventative things you can do so your plants handle the occasional heat wave better.

• Mulch around trees, flowers, and shrubs.  Mulch cools the soil and prevents evaporation
• Water extra during dry periods.  Be sure you are watering the entire root zone of trees and shrubs.  New plantings will require more frequent watering.

Watering Trees

• Mixing compost or humus into the soil when doing new plantings.  The improves sandy soils ability to hold water and clays ability to allow water to penetrate. 

Planting Trees and Shrubs

• Each spring weed and clean out water well apply a 2" layer of compost or mulch.
• Ground covers around trees and shrubs cool the soil and prevents water evaporation.

Ground Covers or Living Mulches

Sweet Woodruff make a beautiful ground cover cooling and protecting the soil.

Ways to help garden plants deal with heat stress:

• You can shade plants with row covers during heat waves.  
• When planning your garden afternoon shade is helpful.  Plants only need 6-8 hours of sunlight.  In our area I highly recommend planting trees on the west side of you garden for late afternoon shade.
• Water deeply not lightly.   The top 1" of soil dries out quickly.  Check moisture by sticking your finger down at least 2 inches into the soil.  The soil should be cool and moist or use a water meter to be sure you are not over watering. 
• Mulch around garden plants with a plant based compost.  This is so important.  It cools the soil, prevents evaporation, feeds soil microbes, and protects against soil borne pathogens splashing on your the leaves of plants.

Building A Healthy Soil

• Spilt up watering times.  So I normally have the water come on in the evening for the garden.  During times of excessive heat and wind I divide the time so water comes on around 12:00 am and 8:00 pm. If you have mulched your garden and used appropriate spacing you will not have water loss during mid day watering.  After the heat wave I go back to the normal watering schedule.
• Frequently check automatic systems to be sure they are working.  Unclog and adjust as needed.
• Check you garden every morning to be sure all areas are being watered.  You will be able to easily see dry spots if you watered the previous evening. This is a good time harvest and monitor for pests and disease.
• Plant vegetable crops in the right season.  Brassicas, spinach, and lettuce are cool season crops and will suffer more in a heat wave.

What to Plant in Early Spring

• Container plants will need daily watering.  If possible move them into the shade. Water container until water can be seen coming our from the bottom of the pot.

Container Gardening

Unique Containers for the Garden

• Prune off heavily scorched leaves and flowers. 
• Avoid over fertilizing or fertilizing during heat waves.  It encourages new growth when the plant is already stressed. Very little fertilizer is needed if you have a healthy soil.

Healthy garden bed with mulch and wood framed raised bed.

Metal Raised beds:

Soil on the edges of metal raised beds will heat up more than in the middle of the deep and more than in wooden raised beds.  Also very tall raised beds will be exposed to more winds.  Wood is my personal preference for a material for raised beds.  I like a bed to be around 12" deep.  Wood will occasionally have to be replaced however; painting with linseed oil extends the life.  

I have a few metal beds in my garden and they never do as well as the wood raised beds.  I do like them for in the greenhouse or if you have an area that receives partial shade.  

Water containers until water runs out the bottom of the container.