Frost in Ireland: what to expect where you live

Irish frosts are mostly light, and many coastal gardens hardly see them. But where you are makes a big difference, and so does the kind of night it is. Here’s how to tell when frost is coming, and what it means for your garden.

Ireland gets off lightly compared with most of Europe at the same latitude, thanks to the Atlantic and the Gulf Stream. Hard, long freezes are rare. But “mild” isn’t the same as “frost-free”. A single clear night in April can still blacken a row of potatoes, or a pot of geraniums brought out a week too early. Knowing when and where frost is likely is one of the most useful things a gardener here can know.

The nights to watch

Most garden frosts in Ireland come on clear, still nights. With no cloud to act as a blanket, the ground loses its warmth straight up into the sky, and cold air settles near the ground. A cloudy or breezy night, even a cold one, is much less likely to bring frost. So if the day has been bright, the wind drops at dusk and the stars come out, take note, especially in spring and autumn.

Ground frost and air frost

Met Éireann forecasts often mention a ground frost. That means the temperature right at grass level drops below zero, even if the air at head height stays just above it. An air frost means the air itself goes below zero. For gardeners, ground frost is the one that matters most. Low-growing seedlings and the new shoots of potatoes and dahlias sit right in the coldest layer. When a ground frost is forecast, cover anything young or tender.

Where you are makes a big difference

Coast versus inland. Gardens near the sea, especially in the south and west, get far fewer frosts than those inland. The midlands and inland valleys get more, and get them earlier in autumn and later in spring.
Hollows and low ground. Cold air is heavy and flows downhill like water. It pools at the bottom of a slope, in a valley or against a solid wall or hedge across a slope. These frost pockets can be several degrees colder than the garden around them.
Height. Higher gardens are colder, and frost can linger there through the day.
Shelter. A bed against a south-facing house wall is the warmest spot in most gardens. An open bed in the middle of the lawn is the coldest.

When to expect the first and last frost

Roughly speaking, the first frosts of autumn arrive from October in colder inland areas, and later near the coast. The last frosts of spring are the ones that catch people out. Inland, they can come well into April and sometimes May, just when garden centres fill up with bedding plants and tomatoes. The usual advice is not to put tender plants out for good until the risk has passed where you live.

If you’d like numbers for your own area, we built a free tool for that. The frost forecast on play.untoldseasons.com shows the frost risk for the next 16 nights for any town, along with typical first and last frost dates worked out from the nights that actually went below zero in recent years. Treat them as a guide. A frost pocket in your own garden can freeze earlier than any forecast says.

What to do when frost is forecast

Cover tender plants before dark with horticultural fleece, an old sheet or even newspaper. Hold it down with stones or pegs so the wind doesn’t take it, and take it off in the morning.
Move pots against the house wall, or into a porch or shed. Group them together for extra warmth.
Earth up potatoes. If early potatoes have shoots showing, draw soil up over them.
Don’t water late in the day in cold weather. Wet soil near the surface freezes more easily.
Let it thaw slowly. Plants frozen overnight suffer most when the morning sun hits them directly. A plant in a spot that thaws gradually often recovers.
Protect the taps. Lag outdoor taps and drain hoses before the first hard frost.

Not all frost is bad

A few frosts over the winter do some good. They break down clods of dug soil into a fine crumb, they help kill off some pests, and plants like parsnips and Brussels sprouts taste sweeter after one. The trick is to know which plants can take it and which can’t, and to be ready on the nights that count. For what else needs doing this time of year, see the Irish garden in December.