Welcome back!
Last time you walked your plot and mapped where the sun and shade fall. Nice work. Today we're adding two more things to that map, wind and cold. Then we'll learn to read a small code on plant labels that tells you whether a plant can get through a UK winter.
Why does this matter? Picture two gardens on the same street. One loses its plants every January. The other gets through winter fine. Usually the difference isn't luck. It's where the plants were put.
Wind: the quiet plant-wrecker
You might expect frost to be the big enemy. The RHS actually says that in Britain, wind does more damage to plants than any other kind of stress.
So what does wind actually do? It dries leaves out and chills them. It can brown the leaves of evergreens, which gardeners call wind scorch. And it rocks plants back and forth, which loosens their roots. That's called windrock.
Where should you look for it? Check these spots:
- Gaps between buildings. These act like wind tunnels. The air speeds up and comes through in sudden, damaging gusts.
- Hilltops and open, exposed plots, including many allotments.
- Coastal gardens, where the wind hardly ever stops.
- Balconies, especially high ones.
Here's a surprise. You might think a solid wall or a close-boarded fence is the best shelter. It isn't. Wind hits a solid barrier, rolls over the top and swirls down the far side in damaging eddies. The RHS recommends a semi-permeable windbreak, like a hedge or slatted fence, that filters out around 50 to 60 per cent of the wind. Think of a sieve rather than a dam. A good windbreak can noticeably reduce wind for a distance of about ten times its height.
Frost pockets: where the cold collects
Now for the cold. On clear, still nights, cold air forms close to the ground. Cold air is heavier than warm air, so it sinks and flows downhill, a lot like water. Where it settles, you get a frost pocket.
So where do you look? Check the lowest part of your plot, the bottom of a slope, and any dip. Also check the uphill side of anything that blocks the slope, like a solid fence, a dense hedge or a wall. Cold air can pile up behind these like water behind a dam.
Why should you care? A frost pocket gets frost later in spring and earlier in autumn, so its growing season is shorter. Early-flowering plants, especially fruit trees, often struggle there because frost kills their blossom.
Here's a tip: go out on a frosty morning. Which patch stays white longest? That's your frost pocket. The RHS also suggests making gaps in a fence or hedge, or thinning its lower growth, so the cold air can drain away.
What about warm spots? A south-facing wall soaks up heat during the day and gives it back at night, which helps keep frost off. That's a great home for the more delicate plants.
Reading an RHS hardiness rating
Pick up almost any plant label and you may see something like H4 or H6. That's the RHS hardiness rating. It runs from H1a (the most tender) up to H7 (the toughest), and it's based on the lowest temperature the plant is likely to survive.
Here's the plain-English version:
- H1a, H1b, H1c: need a heated greenhouse or a spot indoors. Some can go outside in summer.
- H2 (1 to 5°C): tender. Can handle cool weather but won't survive being frozen.
- H3 (−5 to 1°C): half-hardy. Fine in mild or coastal areas, but at risk in a hard winter or from a sudden early frost.
- H4 (−10 to −5°C): hardy in an average winter. Hardy through most of the UK, but not reliably in inland valleys, high ground or colder central and northern areas.
- H5 (−15 to −10°C): hardy through most of the UK, even in severe winters.
- H6 (−20 to −15°C): hardy throughout the UK and northern Europe.
- H7 (colder than −20°C): very hardy, even in exposed upland spots.
So what does that mean for you? If you're a beginner in an ordinary UK garden, H4 and above is a sensible place to start for plants that stay outside all year. Got a frost pocket, an exposed hillside or a garden up north? Go for H5 or higher. Got a balcony? Keep in mind that the RHS says plants in pots are more vulnerable to cold, because their roots aren't protected by the surrounding ground.
One more thing. The RHS calls these ratings a guideline. Many other things affect whether a plant survives, and wind and frost pockets are two of them.
Your frost dates
There are two dates you'll use again and again: the last frost of spring and the first frost of autumn. The time between them is your frost-free growing season.
They change a lot across the UK. The RHS says the last frost usually comes in late spring in the south of England and later in the north and Scotland. Cities and coastal areas tend to clear earlier than inland areas, hills and valleys. Tender plants shouldn't go outside until after your last frost, once nights are reliably above about 7°C.
How do you find your own dates? Look up an online frost-date map for your postcode (some are built from Met Office records), or ask experienced growers at your local allotment. They'll know. And remember, a frost pocket can be colder than the figure for your area.
Your task
- Get out the map you drew last time.
- Mark windy spots with arrows, and shade any frost pockets in blue.
- Circle one sheltered, warm spot, such as the area by a south-facing wall.
- Find your area's typical last and first frost dates and write them at the top.
- Check one plant label and note its H rating.
How to check: Can you point to your coldest spot and say why it's cold? Can you tell whether the plant you checked would be safe outside all year in that spot? If you can, you've got it.
Key takeaways
- Wind is a major source of plant damage in Britain. Watch out for gaps between buildings, hilltops, coasts and balconies.
- Semi-permeable windbreaks work better than solid ones.
- Cold air flows downhill and collects in frost pockets, which have a shorter growing season.
- RHS ratings go from H1a (tender) to H7 (very hardy). For beginners, H4 and above is a good starting point outdoors.
- Plants in pots are more at risk from cold.
- Find out your last and first frost dates. Tender plants go out only after the last frost.