A handful of vegetables consistently underperform in a greenhouse: large vining crops like corn, sprawling winter squash, and most full-size pumpkins take up too much space relative to their yield inside a controlled structure.
The core problem is scale and pollination. A Yardenaler 6×8 greenhouse gives you 48 square feet of interior space — enough for tomatoes, peppers, cucumbers, and leafy greens, but not enough for corn, which requires large block planting for wind pollination to work, or for aggressive sprawlers like giant pumpkins that routinely consume 50–100 square feet per plant. Melons can work in larger greenhouse footprints but need manual hand-pollination since natural pollinators can't reach them inside.
- Corn requires block planting of 16+ plants minimum for effective wind pollination — impractical in any home greenhouse footprint.
- Full-size pumpkin varieties (Atlantic Giant, etc.) need 50–100 sq ft of ground space per plant.
- A Yardenaler 8×16 greenhouse (128 sq ft) is the minimum size where even one melon vine becomes manageable with manual pollination.
- Broccoli and cauliflower bolt faster under greenhouse heat accumulation — field growing typically yields better heads.
- Greenhouse-grown melons and cucumbers require hand-pollination with a small brush since bees are excluded from enclosed structures.