Hardening off tomatoes: why, when & how?

After weeks of pampering in the cosy warmth of a greenhouse, young tomato plants can get a fright if they are exposed to cold weather outside. This is why it is important to harden off tomato plants before moving them outdoors. Learn how in this step-by-step guide.

Hardening off tomatoes: what does it mean?

Hardening off is a way to prepare seedlings for transplanting. Seedlings raised indoors or in a greenhouse are not used to harsher outdoor conditions. As such, young plants need to acclimatise to the extreme day and night temperature changes, lower humidity, wind and direct sunlight before they are moved outside permanently.

By gradually introducing seedlings to the conditions outside over the course of one to two weeks, the plants will thicken and their leaf structure will alter. This process of toughening up and building resistance to the elements is known as “hardening off”.

Why harden off tomato plants?

Hardening off gives tomato plants a chance to adapt to their new environment outside. If you move plants outside too quickly, and do not give them time to adapt, they can experience what is known as transplant shock. Because of the sudden change in conditions, and sun scorching which can result from excessive sunlight exposure, the plants become stunted or, in some instances, stop growing all together.

Although plants often recover from transplant shock eventually, hardening off is better for long-term plant growth and fruit formation.

Tip: Hardening off does not make frost-sensitive plants hardy, so be sure to provide them with protection from cold weather.

When to move tomatoes outside

Late spring is the time to start thinking about moving your tomato plants outside. Begin the hardening off process 7-14 days before the date you plan to transplant your tomatoes outdoors. Only leave your plants outside overnight once the threat of frost has passed: this is around the middle of May in the UK.

Where to harden off tomatoes

A cold frame offers the ideal conditions to harden off your tomatoes, but it is not essential. Any sheltered location outdoors – against a south-facing wall or under a porch – is fine. If you are not using a cold frame, cover your plants with a fleece to prevent sun scorch and temperature shock. Begin by covering the plant with a couple of layers of fleece, and remove each layer gradually over the one to two week process.

Tip: Although tomatoes are sun-loving plants, direct sunlight can be as harmful to seedlings as frost. Be sure to place your seedlings in a shaded spot, or cover them with fleece. Moving the plants out on a dull, cloudy day will also help to reduce their shock.

How to harden off tomato plants: step-by-step

Once you have found a suitable location and your young plants are ready to go outside, simply follow these five steps:

1. Move outside for a short time: On the first day, put plants outside for just an hour or two. Bring the seedlings in at night.
2. Gradually increase exposure: Day by day, lengthen the amount of time your plants are outside. Begin to remove any fleece layers, if you are using them, and expose the plants to some direct sunlight and light breezes, which can help strengthen the stems.
3. Keep an eye on the seedlings: Check the seedlings regularly for leaf curling and wilting. If the seedlings display signs of stress, they either need to be watered or moved back into a shaded or sheltered location.
4. Leave tomatoes out overnight: After about seven to ten days, if the night-time temperature is above 10°C, leave the plants outside overnight.
5. Transplant: Once the plants have been left outside for a few nights in a row, the hardening off process is complete and you can transplant the tomatoes into their permanent spot in your garden.

Tip: If the weather takes a turn and gets colder during the hardening off process, bring your plants back inside until things warm up again.