Lettuce
The anchor crop. Fast rotation, high demand, loves the East Zone’s cooler temperatures. The narrow planting window at this greenhouse is now — by late May, the south zone will be too hot and even the east zone will push lettuce’s comfort zone.
Optimal Conditions
| Parameter | Range | Notes |
|---|---|---|
| Day temperature | 65-75°F | Above 80°F triggers bolting |
| Night temperature | 55-65°F | Cool nights improve crispness |
| VPD | 0.8-1.0 kPa | Moderate — lettuce transpires freely |
| DLI | 14-17 mol/m²/d | Below 12 = leggy. Above 20 = can trigger bolting with heat |
| pH (hydro) | 5.5-6.0 | Slightly acidic |
| EC | 0.8-1.2 mS/cm | Light feeder |
| Photoperiod | 14-16 hours | Day-neutral but benefits from long days |
Varieties for This Greenhouse
| Variety | Type | Days to Harvest | Heat Tolerance | Notes |
|---|---|---|---|---|
| Butterhead (Bibb) | Head | 50-60 | Moderate | Classic, tender leaves |
| Romaine | Head | 55-70 | Good | More heat-tolerant than butterhead |
| Red leaf | Loose-leaf | 45-55 | Good | Fast, cut-and-come-again possible |
| Green leaf | Loose-leaf | 45-55 | Good | Workhorse variety |
| Oakleaf | Loose-leaf | 45-50 | Best | Most bolt-resistant |
Recommendation: Start with a mix of romaine and red/green leaf. Leaf types are fastest and most forgiving. Oakleaf for summer planting — best bolt resistance.
Zone Recommendation
Primary: East Zone hydroponic system
The east zone runs 5-9°F below the greenhouse average during peak heat. On a day when the south zone hits 100°F, the east zone is at 91°F. For lettuce, this difference is the line between a head of lettuce and a flower stalk.
| Factor | East Zone Value | Lettuce Tolerance |
|---|---|---|
| Peak temp (hot day) | ~91°F | Bolts > 80°F sustained |
| Tree shade | Blocks morning direct solar | Reduces heat stress |
| Patio door ventilation | Additional cooling in summer | Helps but introduces dry air |
| Hydro humidity | Evaporation adds local RH | Improves VPD for lettuce |
Seasonal viability:
| Season | Viability | Notes |
|---|---|---|
| March-May | ✅ Excellent | Plant NOW. Prime lettuce window. |
| June-July | ⚠️ Marginal | Even east zone will be hot. Bolt-resistant varieties only. |
| August | ⚠️ Marginal | Monsoon moisture helps but still hot |
| September-October | ✅ Good | Fall crop window |
| November-February | ✅ Good | Grow lights essential; heat cost offset by value |
Hydroponic Growing Notes
- Media: Grodan rockwool cubes → net cups → clay pellets
- Spacing: Every other position (positions 1, 3, 5…) gives 6” spacing for heads
- Nutrient formula: General Hydroponics Flora series, light concentration (EC 0.8-1.2)
- Days to harvest: 45-60 from transplant to hydro (add 10-14 for seedling stage)
- Harvest method: Cut at base for head types. Cut outer leaves for leaf types (extends to 2-3 harvests)
Succession Planting
For continuous harvest, stagger plantings by 2 weeks:
| Week | Action | Positions |
|---|---|---|
| 0 | Seed starts (Jiffy mix, east shelf heat mat) | Tray |
| 2 | Transplant batch 1 to hydro | HYDRO-1 through HYDRO-10 |
| 4 | Transplant batch 2 to hydro, seed batch 3 | HYDRO-11 through HYDRO-20 |
| 6 | Harvest batch 1, transplant batch 3 | Rotate positions |
At 10 positions per batch with 2-week stagger, this yields continuous lettuce with 30 hydro positions dedicated.
Longmont-Specific Notes
- Altitude effect: Higher UV at 4,979 feet can stress lettuce. The opal polycarbonate blocks 99% of UV — this is actually an advantage.
- Dry air: Outdoor RH of 14-18% in spring means the greenhouse VPD will push high. Misters are essential during peak afternoon hours.
- Water quality: Longmont municipal water is soft, low in dissolved minerals. Good for hydro — nutrient solution won’t need pH adjustment from high alkalinity.
→ See All Crop Profiles