Spring greenhouse planning
Build a spring seed-starting calendar that keeps beds full all season
Align seed-starting dates, transplant windows, and greenhouse layout so your crops roll from cool-season harvests into summer production without downtime.
Start with accurate frost data and soil temperatures
Anchor your schedule to the average last frost in your ZIP code, but verify using soil temperature. Most cool-season crops germinate once the top two inches of soil consistently reads above 40°F (4°C). Warm-season seedlings remain under lights until soil warms into the mid-60s.
| Crop grouping | Seed starting window | Minimum soil temperature |
|---|---|---|
| Lettuce & spinach | 2–4 weeks before last frost | >= 40°F (4°C) |
| Brassicas (kale, broccoli) | 6 weeks before last frost | 45–50°F (7–10°C) |
| Tomatoes & peppers | 6–8 weeks before transplant | 70–75°F (21–24°C) under heat mat |
| Cucumbers | 3–4 weeks before transplant | 65–70°F (18–21°C) |
| Herbs (basil, dill) | 4–6 weeks before transplant | 60–65°F (16–18°C) |
Tip: Record soil temperature at 8 a.m. and 4 p.m. daily. The four-day average gives a reliable indicator of when to transplant without shocking seedlings.
Design a spring greenhouse layout that supports staggered harvests
Thoughtful zoning prevents crowding and keeps airflow high as the greenhouse fills. Use these layout principles when planning benches, raised beds, and trellis lines.
Map temperature zones
Group crops by preferred night temperature. Keep heat-loving starts on upper benches or propagation tables and cool crops closer to doors or low benches.
Layer vertical space
Use shelves and hanging rails for seedlings while reserving ground soil beds for quick spring harvests such as radishes and greens.
Rotate by maturity
Pair quick crops (lettuce, bok choy) with long-season plants (tomatoes, cucumbers) so beds stay productive while slow crops establish a canopy.
Need layout inspiration? Pair this guide with the equipment blueprint to coordinate benches, trellises, and sensor placement.
Professional propagation workflow
- • Label trays with crop, variety, and sowing date to avoid mix-ups later in the season.
- • Moisten sterile seed-starting mix before filling trays; avoid compacting cells so roots can breathe.
- • Sow seeds to twice their diameter, then mist gently and cover with a humidity dome or plastic to maintain moisture.
- • Use heat mats for warm-season crops and remove them once seeds germinate to prevent legginess.
- • Brush seedlings lightly or run a fan on low to strengthen stems and reduce damping-off.
Vent humidity domes daily to prevent fungal growth. Once cotyledons appear, remove domes completely and transition seedlings to bottom watering.
Transplant timing that minimises shock
Use the schedule below as your baseline, then adjust based on microclimate data and the crop’s end use (e.g., baby leaf harvest vs. full heads).
| Crop | Start seeds | Transplant notes |
|---|---|---|
| Leafy greens (lettuce, Asian greens) | 4 weeks before target transplant | When 3–4 true leaves; soil 45–50°F (7–10°C) |
| Tomatoes | 6–8 weeks before last frost | After frost danger; soil 60°F (15°C) with row cover ready |
| Peppers | 8–10 weeks before last frost | Soil 65°F (18°C); maintain night temps >55°F (13°C) |
| Cucumbers | 3–4 weeks before last frost | Soil 65°F (18°C); trellis immediately to save space |
| Basil | 6 weeks before last frost | Night temps >55°F (13°C); pinch early to encourage branching |
Environmental management checklist
- • Maintain daytime temperatures between 65–75°F (18–24°C) and nights no cooler than 55°F (13°C).
- • Ventilate during sunny afternoons to keep humidity below 70% and prevent fungal issues.
- • Water in the morning, aiming for evenly moist media without standing water on benches.
- • Install shade cloth or whitewash if interior temps consistently exceed 82°F (28°C) before summer systems are deployed.
Next steps
- Download a frost-date dataset for your county and build a seed-starting spreadsheet.
- Use the growing calendar generator to sequence succession plantings through summer.
- Calibrate thermometers, hygrometers, and grow-light timers before sowing your first trays.
- Schedule weekly crop walks to update transplant dates based on real-time growth.
Frequently Asked Questions
How do I time seed starting to my frost date?
Count backward from your local last frost date using each crop’s recommended lead time. Cool crops can start 2–6 weeks before frost, while warm crops need 6–10 weeks plus protection until soil warms.
Do I still need supplemental light in spring?
Yes if seedlings stretch. Provide 12–14 hours of full-spectrum LED light until days naturally lengthen and light exceeds 350–400 µmol/m²/s at canopy height.
How can I prevent transplant shock?
Harden plants over 5–7 days by lowering greenhouse temps and increasing airflow. Transplant on overcast mornings, water in with a mild kelp solution, and shade for 24 hours if sunlight is intense.