Seed Storage in a Damp Garden Shed

Why damp is the real enemy of stored seed

Seed is alive. Inside every dry little packet sits an embryo waiting for the right combination of moisture, warmth and light to wake it up. In a UK garden shed — particularly an allotment shed with a felt roof and a gap under the door — that combination turns up far too often. Autumn rain, winter condensation and the simple fact that cold air holds less moisture than warm air mean water collects on the inside of the roof, then drips onto whatever is underneath. Once a seed takes on moisture it burns through its stored energy just staying alive, and by spring that packet of parsnip or leek may be fit only for the compost heap.

The ideal conditions for storing seed are cool, dark, dry and steady. Of those four, damp is the one that catches most of us out — and it is the one you can do the most about without spending much at all.

Jars beat packets every time

Paper packets are perfectly good in a shop, where the air is controlled. In a shed they are hopeless, because paper breathes and it draws moisture in. Move your seed into glass. Kilner jars with rubber gaskets are the gold standard, but clean jam jars, coffee jars, spice jars and old honey jars all work well provided the lid seals properly. Test a lid by filling the jar with water, screwing it on and giving it a shake — if it weeps, it will let damp in over a season.

Metal tins and biscuit barrels are a decent second choice, though they can rust from the inside in a damp shed. If you use them, line the bottom with a couple of silica gel sachets and check them after the winter.

Use silica gel — and recharge it

Silica gel sachets are cheap, reusable and the single best addition to a seed jar. They absorb the moisture that gets in every time you open the lid.

• Drop one small sachet into each jar, or two into a large one.
• Keep the sachet in its paper wrapper — never let loose crystals touch the seed.
• Recharge them every few months by spreading them on a baking tray in a low oven (roughly 120°C) for an hour or two, then cooling and returning them to the jars.
• Colour-indicating sachets are handy: they turn pink when spent and blue or orange when they are ready to work again.
• If you want to be properly scientific, a small hygrometer inside your storage box will show you when the reading creeps above about 50% relative humidity.

Label clearly, twice

Mystery seed is wasted seed. Damp air softens biro and smudges marker pen, so label in two places and use pencil or an indelible marker.

• Write on the packet itself: variety, year bought or saved, and where it came from.
• Slip a second card inside the jar with the same details, so if the packet rots you still know what you have.
• Keep the original packet if you can — it carries sowing times, spacing and depth that a handwritten label will not.
• Number your jars and keep a small notebook in the shed listing what is in each one. It takes five minutes and saves a great deal of squinting.
• Add a "sow by" date. Parsnips, onions and sweetcorn are short-lived; brassicas, beans and tomatoes will keep for years.

Outwit mice, heat and sunlight

Mice will chew through paper, polythene bags and cardboard with equal enthusiasm, but they cannot get through glass or a decent tin. That alone is a reason to jar everything up. Beyond rodents, three other things shorten a seed's life:

• Heat. A shed that bakes in July can hit 40°C or more. Keep jars off south-facing shelves and away from anything that generates warmth.
• Direct sunlight. Light degrades seed and heats glass quickly. Store in a cupboard, a closed crate or an old filing cabinet.
• Temperature swings. Steady beats cool-but-fluctuating. The back of a shed, against an internal wall, is better than a shelf by the window.

Never store jars on the floor, where damp rises and mice travel. Raise them on battens, a pallet or a sturdy shelf at least a foot off the ground.

Making a dry corner in a damp shed

If your shed is the only option, build yourself a microclimate. Choose the most sheltered corner, away from the door and any leak. Put a large plastic storage box with a good lid on a high shelf, add two or three rechargeable silica sachets and a hygrometer, and keep your jars inside it. A cool box works beautifully for the same reason. If you have space indoors, a dark, cool cupboard or the salad drawer of a fridge is even better — just bring jars to room temperature before opening them, so condensation does not form on cold seed.

Check your stock twice a year, ideally in early spring and again in autumn. Squeeze a few seeds between your teeth or fingers: brittle and snappy means dry and healthy, bendy means the damp has got in. Recharge the silica, note what is running low, and sow the oldest packets first. A little routine now means strong germination in March — and nothing more disappointing than a row of gaps where the parsnips should have been.