What is a frost pocket and is my garden in one?

A frost pocket is the reason two gardens on the same street can lose a plum crop in different years. It is not bad luck and it is not bad gardening — it is a piece of physics you can see, once you know what to look for, and in many gardens it is genuinely fixable.

Step by step

1. Watch where frost goes last on a bright morning.Go out early after a frosty night and look at the lawn as the sun comes round. Frost that persists in one area for an hour after it has cleared everywhere else marks the pool. Do this two or three times before concluding anything, because sun and shade confuse a single observation.
2. Look at the shape of the ground.Stand at the highest corner of the garden and sight along the ground. Any hollow, dip, terrace or step down is a candidate. So is the lowest corner of a garden that slopes at all, even gently — the slope does not have to be dramatic to move air.
3. Find the barriers across the slope.Walk the downhill boundary. A close-boarded fence with no gap at the bottom, a solid wall, a retaining wall, a garage, or a dense evergreen hedge planted across a slope will all dam cold air. This is the commonest man-made frost pocket and it is usually recent — someone replaced a gappy fence with a solid one.
4. Check the evidence in the planting.In a pocket, spring growth is later, blossom is more often lost, tender perennials fail that succeed elsewhere, and damage sits in a band at a consistent height rather than affecting whole plants. Blossom low on a fruit tree lost while blossom higher up sets fruit is a very strong signal.
5. Decide whether you can drain it.If a barrier is damming the air, opening the bottom of it lets the cold drain away downhill. Removing the bottom board of a fence, cutting a gap through a hedge at the low point, or leaving a gateway open on frost nights can all work, and the effect is immediate.
6. If you cannot drain it, plant around it.Put frost-hardy planting in the pool and the tender, early-flowering things above the level the pool reaches. That means late-flowering fruit varieties rather than early ones, and no tender bedding in the low corner.

Tips

• The fix is counter-intuitive: a solid fence makes a frost pocket worse, and a gappy one or a permeable hedge makes it better. Solid barriers dam cold air exactly as they dam water.
• Cold air needs somewhere to go. Opening a gap only helps if there is lower ground beyond it for the air to flow into.
• A pocket is worst on exactly the nights frost is worst — clear and still. On a windy night the air mixes and the pocket effectively disappears.
• If your whole garden is the bottom of a valley, you cannot drain it and there is no shame in gardening accordingly. Choose late-flowering fruit, hardy planting, and accept a shorter season.

Common mistakes

• Planting an early-flowering fruit tree in the lowest corner because it was the spare space — Early blossom in a frost pocket is the single most reliable way to never get a crop. The blossom opens before the frost risk has passed and the pocket concentrates the frost.
• Replacing a gappy fence with a solid one on the downhill boundary and not connecting the change to the sudden frost damage — The two are directly related and the timing gives it away. A new solid barrier can create a frost pocket in a garden that never had one.

Questions people ask