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Gardening glossary

Stratification

Many seeds will not germinate when first sown even if you give them perfect water, oxygen, and warmth. They are dormant on purpose — a survival mechanism that prevents them from sprouting in late autumn just before they would freeze. To wake them up, you have to convince the seed that winter has come and gone. That is stratification.

The three main types:

1. **Cold-moist stratification** — the most common. The seed is exposed to cold (1–4 °C) and moist conditions for weeks to months. This is the natural overwintering signal for the seeds of most temperate perennials, native wildflowers, fruit trees, and shrubs. 2. **Warm-moist stratification** — used for some seeds with very deep dormancy (paeonies, some hellebores). The seed is held at 20–25 °C and moist for a few weeks before going into the cold treatment. 3. **Scarification** — physically nicking, sanding, or hot-water-soaking a hard seed coat to allow water in. Strictly speaking this is not stratification but it is often paired with it for seeds like sweet peas, lupins, and morning glories.

How to cold-stratify at home:

1. Mix seed with a roughly equal volume of damp medium — milled sphagnum, vermiculite, sand, or moistened kitchen paper. "Damp" means you can squeeze it without water dripping out. 2. Seal in a zip-top bag or small lidded container with the species and date written on it. 3. Place in the back of a refrigerator (not the freezer) at 1–4 °C. 4. Leave for the species-specific duration: 30 days for most native perennials, 60–90 days for many trees and shrubs, up to 120+ days for some stone fruits. 5. Check every two weeks. Any seeds that have already begun to germinate should be sown immediately. 6. When the period is complete, sow as normal and germinate at the species' preferred temperature.

Common seeds that require cold stratification: lavender, milkweed, echinacea, columbine, delphinium, peach, apple, plum, most native wildflowers, most maples, oaks, and conifers. Anything labelled "winter-sown" relies on this mechanism — you can also just sow these in pots left outside over winter and let nature do the work.

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Related terms

Vernalisation

Vernalisation is the physiological cold treatment that biennial and winter-annual plants need before they can flower. Winter wheat, garlic, onions, brassicas, and many bulbs all require weeks of temperatures roughly between 0 and 10 °C to switch from vegetative to reproductive growth.

Germination

Germination is the process by which a seed wakes from dormancy and grows into a seedling. It requires water, oxygen, the right temperature, and for some seeds, light or a cold period. Time from sowing to first true leaves typically ranges from 3 days (radish) to 4+ weeks (parsley).

Dormancy

Dormancy is a plant's built-in rest period — triggered by short days, cold, or drought — when growth slows or stops and metabolism drops. Recognizing dormancy is the difference between a healthy winter pause and a "dying" plant you accidentally drown.

Chill hours

Chill hours measure the cumulative time a fruit tree spends between roughly 32°F and 45°F (0–7°C) during winter dormancy. Deciduous fruit trees like apples, peaches, and cherries need a specific minimum to break dormancy and bloom normally in spring.