Gardening glossary
Harden off
Hardening off is the single most under-appreciated step in vegetable gardening. Seedlings raised indoors or in a greenhouse have soft, thin cuticles, low UV tolerance, and weak stems because they've never felt wind. Moving them straight to the garden bed is like sending an office worker to climb a mountain without training — most will collapse.
A standard hardening-off schedule over 7–14 days:
- **Days 1–2:** 1–2 hours outdoors in dappled shade, sheltered from wind. Bring plants back in before evening. - **Days 3–4:** 3–4 hours outdoors, including 30–60 minutes of direct morning sun. - **Days 5–6:** Half a day outdoors with several hours of direct sun. Light breeze exposure. - **Days 7–9:** Full day outdoors. Start leaving them out overnight if nighttime temperatures stay above 50°F (10°C) for warm-season crops, or above 40°F (4°C) for cool-season ones. - **Days 10–14:** 24 hours outdoors in their final pots, then transplant on a cloudy or overcast day if possible.
Signs hardening off is going wrong:
- **Bleached or silvery patches on leaves** — UV/sun scorch. Reduce direct sun exposure and back off a day. - **Wilting in full sun despite moist soil** — heat stress. Provide afternoon shade for another 2–3 days. - **Snapped stems or torn leaves** — wind damage. Add a temporary windbreak (an upturned crate, a row cover, or shade cloth).
A few shortcuts that don't work: putting them in a cold frame and just opening the lid (still no direct wind), or leaving them on a covered porch (still no UV). The plant has to actually experience the conditions it will live in.
I tell Growli users to start hardening off about 7–10 days before their target transplant date, which itself should sit a week or so after the last spring frost for tender crops.