Kew Gardens Seed Bank Methods for Saving 7 Native Species

Minus 20 in a Chest Freezer: the Wakehurst Number That Transfers

The Millennium Seed Bank Partnership, run by the Royal Botanic Gardens, Kew, keeps close control over moisture content and temperature. Orthodox seeds, the group that includes most native annuals and many perennials, last longest when their internal moisture content sits near 5 percent and storage temperature is held at minus 20 Celsius. At Wakehurst, that means cold rooms and vacuum-sealed jars. At home, the same physics applies on a smaller scale, and even a 4 Celsius salad-drawer fridge buys years compared with a shed shelf where summer heat can climb past 30 Celsius.

Seeds carrying field moisture into a freezer can form ice crystals that rupture cell walls, with viability collapsing on the first thaw. The Kew workflow dries seed against silica gel for one to two weeks until it equilibrates, then moves the sealed material into cold storage. A domestic version needs an airtight jar, 40 to 60 grams of indicating silica per litre of headspace, and enough time for the seed to dry before chilling. Depending on the indicator, the silica changes from orange to green or from blue to clear when it has absorbed its load; a 120 Celsius oven reactivates it.

Foxglove, Field Scabious, Devil’s-bit: Three Orthodox Easy Wins

Digitalis purpurea, the common foxglove, produces tens of thousands of dust-fine seeds on a single spike and stores very well. Collect when the lowest capsules have browned and split before the upper capsules shed, usually in mid to late summer. One spike can fill a paper envelope. After ten days over silica and a move to the freezer, foxglove seed can keep germination above 80 percent for a decade, which explains its regular use in restoration mixes on chalk and woodland-edge sites.

Knautia arvensis, field scabious, and Succisa pratensis, devil’s-bit scabious, follow the same orthodox storage logic, although their harvest window is narrower. Their heads break apart quickly, and a dry breeze can empty them within days of ripeness. The Kew field protocol is to cut whole heads into a paper bag once a gentle tap releases the first few seeds, then finish ripening the material indoors on a tray. Both plants anchor pollinator habitat. Devil’s-bit is the sole larval food plant of the marsh fritillary butterfly, so seed from a healthy garden colony carries conservation value that a catalogue packet lacks.

Kew uses X-ray and cut-test checks on incoming accessions; in a garden, a hand lens gives a workable version of the same judgement. Slice ten seeds and look for a firm white embryo filling the coat. If more than half are hollow, discard the lot. A 50 percent fill rate on the cut test sets a roughly 50 percent ceiling on germination, even if the drying and storage are perfect.

North-Facing Courtyard Ferns Need a Separate Routine

Ferns do not make seeds. They release spores, so the storage routine moves away from orthodox seed banking and toward clean collection with refrigerated paper packets. Asplenium scolopendrium, the hart’s-tongue fern, and Dryopteris filix-mas, the male fern, both suit a shaded north-facing courtyard wall and both shed viable spores from the undersides of mature fronds in late summer.

Cut a frond with brown, plump sori, place it sorus-side-down on white paper inside a closed envelope, and leave it for 48 to 72 hours. The spores fall as a fine brown dust, more uniform than the surrounding sporangial chaff. Stored dry in a labelled glassine packet at 4 Celsius, spores of these species remain viable for several years, although they lose viability faster than orthodox seed.

Fern spores germinate into a gametophyte, a flat green prothallus only a few millimetres across. That stage needs constant humidity and liquid water, because the swimming sperm must reach the egg. Surface-sown on sterilised, lime-free compost in a covered tray and kept under glass at 18 to 20 Celsius, the spores produce a green film of prothalli in four to eight weeks. Recognisable fronds appear months later.

The crevice-planting habit seen in alpine work fits hart’s-tongue fern well. It colonises mortar joints and shaded paving gaps, and young sporelings placed in a vertical lime-mortar crevice match its wild limestone-pavement habitat more closely than they would in a pot.

Fresh Sowing for Oaks, Chestnuts, and Wild Service

Oak, horse chestnut, and the wild service tree produce recalcitrant seed. Drying kills it, so the Kew freezer protocol is fatal for these species. They need fresh sowing, or the storage plan has to change completely.

Heritage Apples Are Kept as Wood, Not Seed

Apples are highly heterozygous obligate outbreeders, so named cultivars do not grow true from pips. A Cox or Egremont Russet pip may produce a tree, yet the fruit will belong to a genetic lottery and is usually poor. The National Fruit Collection at Brogdale Farm in Kent, which holds over 2,000 apple cultivars, keeps named apples through clonal grafting. For a heritage apple, the banked material is scion wood grafted onto a chosen rootstock.

Rootstock determines the final size of the tree and the growing system around it. The East Malling series, developed at the research station of the same name, ranges from dwarfing M27, which holds a tree near 1.5 to 2 metres and suits containers or intensive cordons, through M9 and the semi-dwarfing M26, to vigorous MM106 and MM111 for standards and poorer soils. Ashmead’s Kernel on M27 can fit a small courtyard. The same scion on MM111 makes a 5 metre tree that needs an orchard.

Espalier training brings apples into the scale of a small garden. A maiden whip grafted onto M9 or M26, planted against a south or west wall and pruned to horizontal tiers each summer, uses a vertical plane about 30 centimetres deep and can crop within three to four years. Brogdale maintains worked examples of espalier, cordon, and stepover forms because training shape is where many domestic plantings go wrong. Winter graft wood cut in January, wrapped damp, and kept in a fridge at 1 to 4 Celsius until the rootstock breaks dormancy in spring is the apple grower’s cold-storage equivalent to a seed packet.

Drip Lines and Raised Beds for the Mother Plants

Seed banking starts with a healthy seed-bearing plant. Inconsistent watering disrupts seed set more reliably than storage errors do. A raised bed fitted with a low-pressure drip system using 1.6 litre-per-hour inline emitters at 30 centimetre spacing delivers water to the root zone without soaking foliage, reducing fungal pressure on scabious and foxglove heads during the ripening period. A simple battery timer set to two short cycles a day gives even wetting while letting the surface dry between cycles.

A 1.2 by 2.4 metre raised bed planted with seed-crop perennials needs roughly 20 to 30 litres in a dry week beyond rainfall. A drip line with sixteen emitters rated at 1.6 litres per hour delivers that volume in under an hour of total runtime. During seed ripening, too much water is the more common mistake: capsules that should be drying stay green, and harvest can move past the point where the cut test shows firm fill. Halving drip frequency as the first capsules brown pushes the plant toward senescence and more even seed fill, the same principle Kew uses when accession plants are allowed to dry down naturally before collection.

Labels become part of the living collection: harvest date, drying period, parent plant, and storage location stop a viable batch from turning anonymous. A packet of spores has less context once every shaded joint has been sealed smooth.