Temperature and Light

Each type of seed has an optimum germination temperature. Above or below this soil temperature, the germination rate will be lower, and at some point either up or down, none of the seeds will germinate. 

Here are some optimum temperatures (°C for common veggies):

Broad beans 20, Lettuce 20, Greens 20-25, Broccoli 25, Carrots 25, Pak Choi 25, Onions 25-30, Peas 27, Beans 30, Beetroot 30, Capsicum 30, Cauliflower 30, Corn 30, Cucumber 30, Pumpkins 30, Squash/Zucchini 30, Tomato 30, Cabbage 32, Chinese Cabbage 32, Kale 32, Eggplant 33, Melons 33, Watermelon 33.

Hey wait a minute! Cabbage and Kale are winter veggies, but love 32°C! Yep that's right, and just goes to show that many seeds have quite a wide tolerance. Others, like the heat loving corn, capsicum, eggplant, melons etc, are not so adaptable. Jackie French wrote that it was pointless planting corn seed until the soil was warm enough to allow you to sit on it comfortably with a bare bottom. Sounds like good advice gained by painful experience!

If you are trying to germinate those heat loving plants now, it's probably a good idea to have them inside near a north facing window so the soil can warm up during the day and also avoid the cold outside at night. Even if you don't get frost, they will appreciate missing the wide temperture fluctuations.

When we germinate our heat loving seeds during winter (for earlier spring planting), we use our home made germination chamber. I might tell you about that next week, but for now I want to use it to illustrate a point about light.

Whilst most seeds don't need light to germinate, they need light as soon as they begin to emerge from the soil (or vermiculite). If there is no light, they become spindly and weak, especially if the temperature and humidity are high like in our germination chamber. Look at the poor cucumbers in the photo - even though I check the germination chamber at least twice each day, sometimes they shoot up in just a few hours or overnight, and useless plants are the result. Poor little fella's! I feel responsible for their disability!

For me the solution is to put a grow light in the germination chamber, but for you the north facing window or somewhere else with nice natural light will do the trick. We all need light hey...

Happy week!