Gardening glossary
Day length
Day length (photoperiod) is the most reliable signal a plant can use to track the seasons, because unlike temperature it's perfectly predictable from latitude and date. Many crops have evolved to flower, bulb, or go dormant in response to specific day-length thresholds.
A few latitude reference points (summer solstice / winter solstice day length):
- **Equator (0°)**: ~12h 7m / ~12h 7m year-round - **Miami (25°N)**: ~13h 45m / ~10h 32m - **Atlanta (33°N)**: ~14h 24m / ~9h 54m - **Chicago (42°N)**: ~15h 13m / ~9h 8m - **London (51°N)**: ~16h 38m / ~7h 50m - **Anchorage (61°N)**: ~19h 22m / ~5h 28m
The most practical example for kitchen gardeners is **onions**:
- **Long-day onions** (Walla Walla, Yellow Sweet Spanish) start bulbing when day length hits 14–16 hours — appropriate for the northern US, Canada, and UK. - **Short-day onions** (Vidalia, Texas 1015Y) bulb at 10–12 hours and only work south of about 35°N latitude. - **Day-neutral onions** (Candy, Red Candy Apple) bulb at 12–14 hours and grow across a wider latitude band.
Plant a short-day onion in Minnesota and it bulbs in May while the plant is still tiny — you get a marble. Plant a long-day onion in Texas and it never bulbs at all because day length tops out below the trigger.
Other day-length-sensitive crops:
- **Spinach** bolts (flowers and goes bitter) when days exceed ~14 hours — why summer spinach is so hard. - **Strawberries** split into June-bearing (short-day), everbearing, and day-neutral varieties on the same logic. - **Soybeans, rice, chrysanthemums, poinsettias** all use photoperiod as their primary flowering cue.
Crops controlled mainly by temperature and age — tomatoes, peppers, lettuce, brassicas — are called **day-neutral**.