Silver birch (Betula pendula) is one of the easier trees to raise from seed, as long as you respect two things: the seed is tiny, and it must never dry out. Get the timing, the chill and the sowing depth right, and seedlings can grow fast in their first season.
The short answer: collect silver birch strobiles (the seed-bearing catkins) from late August, just before they turn brown, and dry them in thin layers to release the seed. About four weeks before sowing, soak the seed for 24 to 48 hours, then chill it in moist sand in the fridge. Sow in April, covered by only 1 to 2 mm of grit, and keep it moist in a shady, sheltered spot. The RHS says birch seed germinates at 10 to 15°C (50 to 59°F). Seed-grown trees of named cultivars may differ from the parent.
Before you start: is silver birch right for your area?
Silver birch is the same species the Morton Arboretum calls European white birch, native to Europe and northern Asia. The Morton Arboretum notes it is very susceptible to bronze birch borer and does not recommend planting it anywhere in the Midwest of the United States. If you garden in North America, ask your local extension office whether it is a sensible choice, or consider river birch, which the Morton Arboretum lists as resistant to bronze birch borer.
Step 1: collect the strobiles
Birch seed forms in small cone-like catkins called strobiles. Each one holds many tiny winged seeds. The Conservation Volunteers (TCV) advises collecting the strobiles from the end of August onwards, while they are still intact and just before they turn brown. It warns that the seeds have a high moisture content, so avoid letting collected strobiles heat up in a closed bag.
The RHS notes that seedlings of named cultivars may have different characteristics from the parent, so seed from a cut-leaf or weeping cultivar will not reliably give you the same tree.
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Step 2: dry and store the seed
TCV recommends laying the strobiles out somewhere warm in thin layers, about 10 cm (4 inches) deep, and letting them dry. Once dry, they break up easily and the seeds can be separated. Store the seed in a cool, dry place in plastic or hessian bags.
The RHS adds that birch seed can be sown fresh in autumn or kept in the fridge over winter for sowing in February, but should not be stored for longer than that, because older seed is less likely to germinate.
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Step 3: pretreat the seed
A short, moist chill speeds up germination. TCV’s method:
- Four weeks before sowing, put the seed in a cotton bag and soak it in clean, cold water for 24 to 48 hours.
- Dry the surface of the seed. TCV suggests spinning the tied bag briefly to remove excess water.
- Mix the seed with a little moist horticultural sand and put it in loosely tied plastic bags in the bottom of the fridge, not the freezer.
- Leave it there for about four weeks. TCV says this improves the speed and evenness of germination.
Step 4: sow it barely covered
Depth is where most birch seed fails. Both sources stress a very light covering:
- TCV says to sow in April on seedbeds or in pots, 5 to 10 seeds per pot, and cover with only 1 to 2 mm of coarse horticultural sand or grit. Covering the seed too thickly can prevent germination.
- The RHS says to sow thinly on the surface of peat-free seed compost and cover very lightly with more compost, then water gently.
Firm the seed gently after sowing.
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Step 5: keep it moist
TCV’s warning is simple: never let the seeds or young seedlings dry out. Because the seeds are so small, they suffer quickly without water. TCV suggests standing pots in a shady, sheltered spot outside and watering from the top until seedlings emerge. The RHS says seeds need 10 to 15°C (50 to 59°F) to germinate and should be watered regularly and given good light.
| Stage | What to do | Source |
|---|---|---|
| Late August onwards | Collect intact strobiles just before they brown | TCV |
| After collection | Dry in thin layers; store seed cool and dry | TCV |
| Four weeks before sowing | Soak 24 to 48 hours, then chill in moist sand | TCV |
| April (or autumn, if fresh) | Sow, covered by 1 to 2 mm of grit | TCV, RHS |
| Until emergence | Keep moist; 10 to 15°C | TCV, RHS |
Step 6: raise the seedlings
Once seedlings have two leaves, the RHS recommends moving them into individual small pots, then potting on regularly so growth is not checked. If you sowed several seeds per pot, TCV suggests removing all but the strongest seedling.
TCV recommends feeding regularly with a liquid fertilizer until August. It says up to 60 cm (about 2 feet) of growth in the first year is not unreasonable, and plants can stay in pots for a second year if fed while actively growing, April to September.
Step 7: plant it out
When the young tree is big enough, plant it in a cleared spot. TCV advises removing weeds and grass, digging a hole big enough for the root ball and planting at the same depth it grew in the pot. The International Society of Arboriculture also recommends a hole two to three times wider than the root ball, with the trunk flare partly visible after planting.
The Morton Arboretum lists European white birch as a 40- to 50-foot tree that grows in full sun or partial shade and prefers moist, well-drained soil, so give it room. Birch roots are shallow and wide, which you can read about in how deep birch roots go, and young birches need steady moisture; see how much water birch trees need.
Sources
- How to grow Silver birch, The Conservation Volunteers (TCV), How to Grow Trees
- How to grow birch, Royal Horticultural Society
- European White Birch, The Morton Arboretum
- River Birch, The Morton Arboretum
- Planting a Tree, International Society of Arboriculture, TreesAreGood.org
Top photo: Krzysztof Ziarnek, Kenraiz, CC BY-SA 4.0, via Wikimedia Commons