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Clethra alnifolia

Coastal Sweet Pepperbush · Pink-spire · Summersweet · Coastal Sweetpepperbush

Family ClethraceaeGenus Clethra

An upright spike of small white flowers with long stamens, opening from pink buds at the tip
FlowerSten PorseCC BY-SA 3.0

About the plant

What it is and where it grows — not part of any germination protocol.

The plant

Habit
shrubPlants of the World Online
Habit
Perennial, ShrubPLANTS Database
Height
5-10 ft1.5-3 mExtension Gardener Plant Toolbox
Spread
4-6 ft1.2-1.8 mExtension Gardener Plant Toolbox
Flower
Pink, WhiteExtension Gardener Plant Toolbox
Blooms
SummerPLANTS Database
Native range
Canada, lower 48 statesPLANTS Database
Hardiness zones
3-9Extension Gardener Plant Toolbox

Growing conditions

Sun
Full sun; Partial ShadeExtension Gardener Plant Toolbox
Soil
Clay, High Organic Matter, Loam (Silt), SandExtension Gardener Plant Toolbox
Soil pH
Acid (<6.0), Neutral (6.0-8.0)Extension Gardener Plant Toolbox
Moisture
Good Drainage, Moist, Occasionally Dry, Occasionally WetExtension Gardener Plant Toolbox
Watering
Drought tolerance: LowPLANTS Database

Toxicity is noted only where a source states it. No note here is not a finding of safety.

Sources: Plants of the World Online — Royal Botanic Gardens, Kew; PLANTS Database — USDA NRCS; Extension Gardener Plant Toolbox — NC State Extension.

Worth knowing

This source reports two treatments. Both are shown.

Protocol 1

DerivedN. C. Deno, 1993

Difficulty Moderate Rated by us, not by the source.

Start around December 15, 2026 for a May 3 last frost.

  1. Cold moist stratification

    Duration: 90 daysTemperature: 40°F 4°C

  2. Warm moist stratification

    Duration: 90 daysTemperature: 70°F 21°C Light: light

about 3 months from start to step 2. Germination happens during step 2, in 21 days.

60% germination in 21 days.

Seed Germination Theory and Practice, 2nd ed. — N. C. Deno, 1993 · Ch. 20, Clethra p. 121Public domain
Show the original text
Clethra (Cletharaceae). Both species required light for germination. C. alnifolia germ: 40-70L(60% in 3rd w), 40-70D(none), 70L(10% in 5th w), and 70D(none). Seed DS 6 m at 70 gave the same results, but seed DS 12 m was dead.
Seed Germination Theory and Practice, 2nd ed., Ch. 20, Clethra p. 121. Quoted exactly. Every value on this card comes from this record, except the difficulty rating, which is ours; the step names come from a fixed list.

Protocol 2

DerivedN. C. Deno, 1993

Difficulty High Rated by us, not by the source.

Start around March 1, 2027 for a May 3 last frost.

  1. Sow and cover

    Temperature: 70°F 21°C Light: light

No stated waiting before sowing. Then 35 days to germinate.

10% germination in 35 days.

Seed Germination Theory and Practice, 2nd ed. — N. C. Deno, 1993 · Ch. 20, Clethra p. 121Public domain
Show the original text
Clethra (Cletharaceae). Both species required light for germination. C. alnifolia germ: 40-70L(60% in 3rd w), 40-70D(none), 70L(10% in 5th w), and 70D(none). Seed DS 6 m at 70 gave the same results, but seed DS 12 m was dead.
Seed Germination Theory and Practice, 2nd ed., Ch. 20, Clethra p. 121. Quoted exactly. Every value on this card comes from this record, except the difficulty rating, which is ours; the step names come from a fixed list.

How each step is done, in Deno’s words, and what Verified and Derived mean.

Germination trials

Readers’ own records of sowing this species, published after review. Share your trial · All trials

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