Are we there yet? When to plant tomatoes…

soil thermometer with blue string

Spring is the time of year in Southern Australia when gardeners are desperate to plant their tomatoes, but it really depends on the soil temperature in your garden and whether you are in an area which can be affected by frost.

Tomatoes should not be planted until the soil temperature reaches a minimum of 16 degrees, and 18-20 degrees would be even better.  The same is true for eggplants, chillis, capsicums, basil, squash, pumpkin and zucchini.  Melons and cucumbers need even warmer soil and do best sown into soils with temperatures of 18-22 degrees.

So how do you check the temperature of your soil?

Well you get a soil thermometer, and having lost several of these in the past two years (when placed in the soil and then I forgot where I have put them), I have attached a piece of bright blue baling twine to the end of my latest.  You could also go with the layman’s rule of thumb for 16 degrees soil temperature – when you can comfortably sit both bare cheeks on the soil.   It is also the same for citrus which shouldn’t be planted out or repotted till the soil temperature is at least 16 degrees.  Planting citrus too early will actually set them back and it can take them month and sometimes years to recover.

Frost protection

At Sophie’s Patch, even when the soil temperature gets warm enough, frosts can kill young plants so it is essential in frosty areas to keep the newly planted vegies warm and protected from cold weather especially chilly nights by using clear plastic shelters known as cloches over individual plants, which will keep the plant warmer but not affect the soil temperature.  You can make a simple cloche from a plastic soft drink or milk bottle with the bottom cut off and the lid left off.  Alternatively, make a mini poly tunnel over the whole bed with plastic sheeting stretched over a poly pipe or metal frame – this will not only trap warmth in the vicinity, it will help to increase soil temperature.  I have at times used glass flagons which have had their bottoms cut out to cover my young tender seedlings and protect them from frost, mulch like they do with the glass cloches in the UK.  However, when I forgot to take them off on a warm day, the seedlings cooked, so be warned.

Another option in cold frosty areas like here at Sophie’s Patch is to grow your plants like tomatoes on in into larger plants by potting them on, so when the soil is warm enough and the risk of frost is over, that is when you plant them out and they are already more established.

When do I plant at Sophie’s Patch?

The current soil temperature in vegie beds at Sophie’s Patch (in September 2017) is only 12 degrees so I will have to wait a while longer.  The last frost here can be as late as early November so I will definitely leave it for a few more weeks before I start to plant, and then try and be vigilant with frost protection on clear nights.

Scroll to Top