Growing Tomatoes in your Greenhouse

Tomatoes are generally a pretty easy food-producing plant to grow. They also have endless varieties in cooking and can be easily preserved by canning into diced tomatoes, sauces, salsas, and more. Learning to grow tomatoes is worth the effort, especially when they can reach prices of up to $15/kilo in the off season in New Zealand!

The chart above gives you a quick reference as to monthly temperatures, approximate greenhouse temperatures and the ideal tomato germinating and growing temperatures.

As you can see from the chart below, December, January, February and the beginning of March are the best times to grow tomatoes outside. If you are wanting tomatoes in the colder months, it is definitely possible with a greenhouse and a little effort to create the right conditions.

Most tomato varieties don’t like to be colder than about 15°C. The end of February nighttime temps can start dropping below 15°C. This starts becoming a problem when it stays under

15°C for longer than a week.

The heat inside a greenhouse can be anywhere from 5° to 15°C warmer than outside, without adding a heat source like a space heater. This can vary depending on greenhouse location and type of glazing. Glass has a lower thermal rating than polycarbonate, hence more heat transfer happens in a glasshouse than a polycarbonate greenhouse.

Depending on the temps in your greenhouse, you may need to bring in an additional heat source if you are planning to grow tomatoes in the coldest months and shade cover to cool off in the heat of summer, unless you are moving them outside for the summer.

Aside from temperature, pests and mildew are another greenhouse growing consideration. To detract white flies, you can plant companion plants like Basil or other strong-smelling plants. You can also use sticky traps to catch white fly and other pests, which you can buy or make yourself.

If you want to make sticky traps yourself, use a 1:1 ratio of water to corn syrup. Boil it gently, then cool completely. Then use yellow colour paper (1/4 pieces of an A4 paper with a hole punched for hanging) and paint your ‘glue’ mixture onto the paper. Hang the sticky papers in your greenhouse and change out once a week or so as you see them fill up with white flies.

In spacing the plants, make sure the plants are at least 30cm apart to avoid the leaves touching neighbouring plants too much.

The leaves are susceptible to mildew if they stay wet, so once the plant has a solid stem, pinch off any leaves that are touching the ground. Speaking of pruning, you’ll also want to pinch off the suckers, those little side growths that grow out of the plant’s ‘elbow’.

Make sure your greenhouse is well-ventilated for good air circulation. This is good for both avoiding pests and disease as well as pollination. Open all doors and vents when the weather is nice. If you have auto vents, they will do this automatically. You can also bring in a fan to help circulate air.

If you are keeping your tomatoes in your greenhouse for their whole life cycle, you will need to help them pollinate. Tomatoes grown outside can be pollinated by bees or other insects, or even just the wind. But in a greenhouse, there’s no wind. So you can either bring in bees, or hand pollinate the flowers.

To hand pollinate, use a a tiny brush or cotton swab to pick up pollen from the anthers and deposit it on the stigma. Alternatively, tap the blossom with a pencil or chopstick to shake the pollen onto the stigma. You can also use a vibrating toothbrush for this. Hand pollinating can be time-consuming but worth the reward of fresh tomatoes in the winter.

Keep your tomatoes well fed with either a store-bought tomato fertilizer, or a homemade one. There are too many recipes to include here, but a quick Google search will reveal many different fertilizer recipes that gardeners swear by. Epsom salts, milk-powder and seaweed are all popular tomato foods. More important than which you choose is consistently feeding them every week. The more you feed the more they produce! Happy year-round growing!