Today we will dive deeper into the mysteries of the right temperature for the right vegetables. I want to talk to you about the right temperature for starting, growing, watering, and harvesting your vegetables. I’ll also explain why this is the case and how you can take advantage of this knowledge. Let’s get started.
The Basics
Temperature is very important when growing vegetables and can sometimes be forgotten completely. All vegetable seeds and seedlings require a certain minimum and maximum temperature to start the germination process. When germinated they want a specific growing temperature. And later on, when harvesting we can actually improve the storage time of our vegetables by harvesting at the right temperature and moisture levels.
There are of course many variables when dealing with your climate and sudden hot weeks in the garden. But generally plants will thrive if temps go up and down gradually. We can protect our plants with some extra shade in sudden hot days.
Measuring Soil Temperature
Measuring the soil temperature can be done by inserting a soil thermometer. Going down anywhere between 1 and 4 inches deep we can measure the temperatures. When testing out the temperature of your soil make sure to take an average of at least 3 days. You can imagine measuring on just your hottest day might give a different result.
Go out in the morning and stick the probe into the soil and wait a couple of minutes so the thermometer can adjust. I like to take a sample from different soil levels. Some plant roots are deeper than others so it’s good to know the averages going down.
I know what you are thinking… Great now I know the soil level but what can I do with that information? Well, it depends… If you have a garden it might be some areas are shaded by trees. Or maybe you are growing some tomatoes or peas that create a shadow at some point during the day. By testing out different locations in your garden you will figure out your shadow plan. Some vegetables or flowers prefer to be in some shade. So we can use this plan to our advantage and plant accordingly.
Germination Temperatures
If we look at germination rates we know that there are minumum and maximum temperatures. Going above or below this might stun or kill the seedling. So by testing our soil temps we also know if we maybe need to add more shade to some trays or put them in the sun or even inside on a heat mat or window sill.
As you can tell there are many good reasons to test out your soil temperatures. Did you know germinating your seeds at optimal temperatures can speed up the germination time massively? For some seeds, we can half the time it takes for them to start sticking out their first leaves. Here is a list with all optimal germination conditions for all seeds.
Vegetable Germination Temperatures
| What | Min °C (°F) | Optimal °C (°F) | Maximum °C (°F) | Germination time (days) |
| Asparagus | 10 (50) | 24 (75) | 35 (95) | 14 – 18 |
| Aubergine/Eggplant | 15 (60) | 30 (85) | 35 (95) | 12 – 20 |
| Beetroot | 8 (46) | 28 (82) | 33 (90) | 4 – 10 |
| Broccoli/Calabrese | 7 (44) | 25 (76) | 30 (85) | 7 – 12 |
| Brussels Sprouts | 6 (42) | 30 (85) | 35 (95) | 3 – 10 |
| Broad Beans | 15 (60) | 27 (80) | 30 (85) | 4 – 10 |
| Cabbages | 10 (50) | 30 (85) | 35 (95) | 5 – 10 |
| Carrots | 7 (44) | 25 (76) | 30 (85) | 6 – 10 |
| Cauliflower | 8 (46) | 23 (73) | 30 (85) | 4 – 10 |
| Celery | 15 (59) | 20 (68) | 24 (74) | 10 – 12 |
| Cucumber | 20 (68) | 30 (85) | 35 (95) | 5 – 8 |
| Endive | 1 (35) | 21 (70) | 30 (85) | 10 – 14 |
| Kale | 10 (50) | 32 (90) | 37 (98) | 5 – 7 |
| Leek | 8 (46) | 25 (76) | 30 (85) | 8 – 16 |
| Lettuce | 7 (44) | 18 (64) | 23 (73) | 2 – 10 |
| Onions | 10 (50) | 23 (73) | 30 (85) | 4 – 12 |
| Parsnip | 10 (50) | 17 (62) | 20 (69) | 5 – 28 |
| Peas | 15 (60) | 23 (73) | 27 (80) | 5 – 7 |
| Peppers | 18 (65) | 30 (85) | 35 (95) | 7 – 10 |
| Pumpkin | 15 (60) | 33 (90) | 38 (100) | 4 – 10 |
| Parsley | 10 (50) | 23 (76) | 30 (85) | 5 – 6 weeks |
| Tomatoes | 5 (40) | 27 (80) | 33 (90) | 5 – 7 |
| Turnip | 15 (60) | 30 (85) | 38 (100) | 3 – 10 |
| Radish | 5 (40) | 30 (85) | 35 (95) | 4-10 |
| Corn | 10 (50) | 35 (95) | 40 (105) | 4 – 10 |
| Spinach | 1 (35) | 21 (70) | 30 (85) | 6 – 14 |
| Chard | 5 (40) | 30 (85) | 35 (95) | 7-10 |
| Okra | 15 (60) | 35 (95) | 40 (105) | 7 – 12 |
| Squash | 15 (60) | 35 (95) | 38 (100) | 7 – 10 |
| Watermelon | 15 (60) | 30 (85) | 40 (105) | 4 – 10 |
Growing Temperatures
Now that we know the right temperatures to start the germination process let’s look at the growing temps. Usually, plants will do just fine in many temperatures. Some might start flowering when getting too much or too little heat or die off when getting too little. Crops like Beetroot, cabbages, carrots, broccoli, and Spinach could be stimulated into flowering if they reach lower than 7°C (44°F) for a longer period. Lettuce on the other hand doesn’t like temperatures above 30°C (85°F). Vegetable crops like tomatoes or beans and peas might drop some of their flowers if the temps are not right, resulting in less harvest. Some crops even only produce male flowers in high temperatures affecting pollination.
Knowing what temperature is right for your crop will help you determine the layout for your garden and when to start what plants. In general, plants are quite forgiving. That’s why I want to look at the optimal temperature.
So next time you are in the garden take out some time to think about the temperature of your soil and what plant should grow where.