Tough Love for Tender Plants: Mastering Hardening Off before planting out tomatoes

British Tomato Fortnight

British Tomato Fortnight this year is from 1st June to 14th June and is a great time to focus on British Grown Tomatoes and this year promotes tomatoes as a good go to snack. Head to their website at www.britishtomatoes.co.uk to see how different growers aim to provide an extended seasonal british crop using many more sustainable ways of tender plant farm production which minimises transport costs by being locally grown. Of course if you grow your own then you have significantly reduced growing to tasty treat miles. Who doesn’t like a sneaky snack of a sun warm ripe tomato? 

So how can we get the best out of our tomato plants at this time of year? Tomatoes are tender plants requiring frost free conditions. They need warmer temperatures - consistently between 15 and 30 deg C to germinate and grow best if the temperature doesn’t fall below 15 to 18 deg C overnight with warmer temperatures in the daytime. If they suffer a temperature shock with temperatures being less than 12 deg C then this will stunt growth or at worst kill the plant. This means care needs to be taken as to when to plant them in a cool greenhouse, patio or outdoor bed.

So how do you decide when to plant out your tender plants such as tomatoes, begonias, petunias, peppers, courgettes, french dwarf and climbing beans and cucumbers?

Generally air temperatures need to be consistently above freezing at night and during the day. Be aware that although daytime temperatures may look reasonable at 12-14 deg C then as I found out by putting my small tomato plants out in a sunny position on a cooler, but windy day the leaves still got wind and probably cold burnt and they have taken a while to recover. Note how the lower leaves have browned and are shrivelling.

Hardening off 

Getting young plants that have been sown and grown indoors used to the outside conditions is called hardening off. The aim of this is to expose them to outdoor conditions such as wind and rain but protect them from temperatures that are low enough to stunt their growth or so low that they are killed off. 

Hardening off takes 2-3 weeks before they adjust to the outside conditions. Start off with putting them out in a sheltered position during the day and bring them in at night. Alternatively a cool greenhouse or a cold frame can be a way of acclimatising them increasing the ventilation over the first couple of weeks. During this time keep a check on the weather forecast for colder days and nights and adjust accordingly when you put the plants out. If they are nearly ready to plant out and a cold night is forecast then a single or double layer or fleece will help protect overnight. In most parts of the UK tender plants can be planted out at the beginning of June so can start hardening off in mid may. This varies depending on which part of the country you are in and on local microclimates.

Last frost days

You may see information about last frost dates. These are an average as to when the last frost will occur in your area. It needs to be remembered that as an average it does not show which year you will get a later frost than average - so keep an eye on the local weather forecasts and if it looks likely for temperatures to fall then add a couple of layers of garden fleece over them to protect.

What affects last frost days?

In the UK where you are located north to south tends to affect how late you will experience frost.

• Altitude likewise means that temperatures are cooler the higher up you live.
• Coastal areas are more likely to have milder temperatures than more central or exposed areas.
• Wind is another factor to consider - as we know the temperature feels chillier if there is a wind blowing.

The last two weeks in our part of North Wales have seen wide variations in temperature like many other parts of the UK. I have been checking my Maximum and Minimum thermometer in the greenhouse to work out when I can plant my tomatoes there. I am a somewhat cautious gardener and in checking the temperature have delayed planting up my tomatoes in my cool greenhouse and my winter squash in the bed outside..

A Maximum-minimum thermometer keeps a record of the current temperature, the maximum that has been reached since its last reset and likewise the minimum since last reset. I am a  bit cautious so I have delayed putting my tomatoes out.

You may be interested to see the variation in temperature I have recorded in my greenhouse over the last couple of weeks. During the previous week I several times saw night time temperatures of around 4-5 deg C. Note that higher daytime temperatures do not necessarily mean a warmer night time one.

Temperatures in deg C

For further information

https://www.plantmaps.com/interactive-united-kingdom-last-frost-date-map.php

https://www.rhs.org.uk/garden-jobs/when-to-plant-out-tender-plants-avoiding-late-frosts