Frost occurs when the temperature of the air or the ground falls below zero degrees Celsius. Gardeners need to know the last and first frost dates of the year in order to protect tender plants. The last frost date refers to the final spring frost of the year; the first frost date refers to the first frost in the autumn.
When will it be?
It’s the middle of May and the weather is warming up, so we can potentially start planting tender annuals outside. Seed packets for annual fruits and vegetables often say things like ‘sow 10 weeks before the last frost’ or ‘plant out when frosts are over’. But, how do we know when the last frost has been and gone?
Images by Inn and Peggychoucair from Pixabay
The last (and first) frost dates vary each year and the average dates are different across the country, so there isn’t a single date you can use. The date depends on a number of factors like whether you are:
- in the North or South of the country
- on a hill or in a valley
- in the middle of a town or in a more rural location
- near a lake or the sea
In the UK, the last frost date ranges from the end of March to the middle of May.
You might have heard the phrase ‘’Ne’er cast a clout till May be out”
A ‘clout’ is an Old English word for a piece of cloth or clothing. The phrase is warning against stripping off your winter clothes too early.
There is debate about the meaning of ‘till May be out’. It might refer to the month of May, or it could be a reference to when the Hawthorn (or May flower) blossoms. Read more about the history of the phrase here.
Image by Steve Bidmead from Pixabay
Although not specifically a gardening phrase, it indicates that the weather can be very variable and there can be surprising cold spells until the end of May. In 1936, there was a frost in Rickmansworth, Hertfordshire as late as the 29th May1
There are lots of websites where you can look up the average last frost date in your town. For Oxford, some sites said 21st-30th April2, whereas one said the second week of May3. Looking at the forecast, it is probably safe to start planting out now, but perhaps save a few plants just in case.
It’s worth keeping your own log of first and last frost dates at your garden or allotment – this will be even more accurate than an Oxford average.
Why does it matter?
Plants can generally be divided into ‘hardy’ or ‘not hardy’, also known as ‘tender’. Hardy refers to plants that can tolerate low temperatures and survive. Many of the vegetables and flowers we grow are tender annuals. This means that low temperatures and frosts will either stop their growth, damage or even kill them.
What if I get it wrong?
All is not lost. If you have merrily planted your tomatoes outside and then you hear that there will be an overnight frost, there are ways to protect your plants.
Keep horticultural fleece at the ready to drape over plants – hold it in place with stones or bricks.
You can also use ready made plastic or glass cloches. Cloches are portable covers and come in a bell shape to cover individual plants or as tunnels to cover a row of plants.
Make your own by cutting the bottom off plastic bottles – remember to take the lid off to allow ventilation.
If you have started your seeds indoors or in a greenhouse, then they need to slowly get used to life outdoors, so that they are not shocked by cooler temperatures, wind and rain. This is called ‘hardening off’.
It’s important to note that this process will not turn a tender plant into a hardy one, so you still need to keep an eye on the forecast in case of frosts.
A cold frame is a good way to slowly accustom plants to being outdoors. This is a frame structure made of wood or metal with a plastic or glass lid that can be removed or held open. You can buy them or try making one with some old timber and perspex or even old windows.
Keep the lid closed at first and then open it on dry days, gradually keeping it open for longer periods and eventually overnight. Close it up again if there is a frost forecast – you might even want to put a layer of fleece on top. After around a week with the lid open at night, you can move the plants to their final position in the ground.
Bottom image by Bernadette Kaufmann from Pixabay
If you don’t have a cold frame, you can move young plants outside in a sheltered position, out of the wind and direct sunlight. Start by leaving them out for a few hours during the day and bringing them back in at night. Once nighttime temperatures increase, start leaving them outdoors all the time, unless frost is forecast. Plant them in their final locations after another week or so.