by Lois Scott, Master Gardener

Excessive heat can certainly make it hard for me to work in the garden.  It can also affect how your vegetables work.

For most plants ideal growing temperatures range from 15 to 30 degrees Celsius (C).  When temperatures are above 32 degrees C for long periods of time plant growth is slowed and some plants will begin to show stress.  This link describes a number of heat-related issues and some solutions. 

A neighbour and new gardener commented that he wasn’t getting any fruit forming on his only squash plant and wondered if he had a plant with only female or male flowers.  I reassured him that squash plants and other cucurbit vegetables like cucumber and zucchini all produced male and female flowers on the same plant. 

While this is true, apparently daytime temperatures of 32 degrees C and up and night time temperatures of over 21 degrees C can change the ratio of male to female flowers to more male than female flowers.  These same temperatures can also cause flowers and small fruit to drop.  Too much or too little water can also cause flowers and fruit to drop. 

Watering may be within our control but temperatures may not be!  There are varieties that are touted to be heat tolerant and could be a good option for our increasingly hot summers.

Another cause of no fruit or deformed fruit is poor pollination.  Apparently, many bees don’t like to work when it’s hot.  Temperatures over 32 degrees C causes many bees to slow down and pollinate less. 

Tomatoes may also be affected by very hot weather.  During hot spells with daytime temperatures above 29 degrees C and night time temperatures above 21 degrees C, tomato plants may become stressed, depleting the plants energy stores and changing the flower making it harder for them to be pollinated.  The flower will then drop from the plant.

Heat may also keep your tomatoes from turning red.  Tomatoes ripen in two stages.  They are green, seeds form, the area around the seeds becomes soft and gelatinous and they are mature.  Then they turn red.  The ideal temperature for turning red is 20 to 25 degrees C. The pigments that turn mature tomatoes red are not produced when the temperature is above 29 degrees C. 

The good news is that when the temperature is ideal, they will turn red. More here.

Gardening provides lots of learning opportunities to improve outcomes but sometimes the weather gets in the way!