Seed Germination & Soil Temperature
By Cheryl Discenza
Seed Planting Season
It’s almost that time! Many gardeners will soon be gathering their seed growing supplies to germinate their favourite vegetables, herbs, and annuals. There are many aspects for successful seed germination, including both chemical reactions and physical reactions. For example, the seed absorbs the water which triggers a variety of chemical reactions that activate enzymes and hormones to stimulate seed growth. Viable seeds require water, oxygen, sometimes light (or darkness), and must have optimal temperatures for successful germination.
How Does Temperature Affect Seeds?
Soil temperature is a key factor that either allows for successful germination or prevents it completely. The chemical and physical reactions cannot take place if the temperature is too hot or cold. Most seeds will not germinate below 0°C or above 40°C. If the temperatures are within an acceptable range but not at their optimal temperature range, they may develop perfectly fine, however, it also means there is a greater chance that growth will be slow or stunted. This will cause a weaker seedling or eventual failure of the seed to germinate. For many plants, seeds will not be successful below 4°C-10°C. The seeds may absorb water but will not start developing which will cause the seeds to become diseased or rot. However, there are some plant species that require cold stratification to germinate.
What is an optimal temperature for seed germination?
Optimal temperatures for seed germination are the temperatures at which the seed develops the fastest and have greatest success. The rate of success is based on the number of days the seeds take to germinate. Most crop seeds germinate with a temperature range of 18°C to 29°C, however, all seeds are created differently and therefore the optimal temperature will vary depending on the type of seed. Cool season plants, such as leafy greens, prefer a lower optimal temperature of 7°C to 24°C. These plants can be directly sowed outdoors in early spring. Warmer season plants, such as peppers, have a higher optimal temperature of 29°C. Home gardeners will usually start these seeds indoors in early spring. Heating mats are ideal to raise the soil temperature for the seeds when starting indoors. It is recommended to research the seed you are planning to germinate and determine the optimal temperature range.
How to determine the soil temperature
Most seed packets will offer detailed information of when to start your seeds, whether it’s indoors or outdoors. This is the best reference for when planting your seeds. When directly sowing the seeds outdoors it may be more challenging to know when to plant. Different areas in our yards will probably have different soil temperatures. The soil temperature can be influenced by a few different factors including whether there is mulch on the soil, the location of the soil, and if the location is shaded. Bare soils warm up faster compared to soils covered with mulch. An unobstructed south facing garden will have warmer soil temperatures as it receives longer and more intense sun compared to north facing gardens. One way to determine the soil temperature is to insert a thermometer into the soil at the depth of where you would plant the seed.
The soil temperature is a key factor in seed germination. The temperature will determine the number of days it takes for the seed to sprout through the soil. Seeds have a high success rate if they are viable and given the necessary conditions to grow, including the optimal soil temperature. Always determine the seed type and it’s growing requirements for successful germination. I have heard people say they cannot grow plants from seeds; however, everyone can be a successful germinator, just remember, some seeds like it hot.
Note: Native Plants & Seed germination
You may have heard of cold stratification before, which applies to native plant seeds. Many of our native plant species require a period of cold/moist temperatures to trigger the chemical and physical reactions in the seed that is necessary for it to germinate. Years of coevolution between our native plant species and climate have created the perfect balance so that the seeds do not start growing on balmy November days, but instead, go through our cold winters and are ready to sprout in the spring.
- Empress of Dirt: Best Soil Temperatures for Sowing Vegetable Seeds, October 6, 2021, Melissa Will
- Burlington Community Gardens: Seasonal Vegetable Planting Guide
- Alabama A&M Lawn and Garden: Soil Temperature Conditions for Vegetable Seed Germination
- Permiculture Research Institute -Soil Temperature and Seed Germination, Paul Alfrey Paul Alfrey March 9, 2017