Yes, garlic, a long-season crop, can be successfully cultivated in a greenhouse, primarily to extend the growing season or provide necessary climate control. Growing this bulb indoors allows for protection from harsh weather and pests, but it demands precise environmental manipulation.
Creating the Required Cold Period
Garlic must undergo a period of cold exposure, known as vernalization, to stimulate the plant to form a segmented bulb with individual cloves instead of a single, non-dividing “round.” The cold treatment is typically needed for 6 to 10 weeks at temperatures between 32°F and 50°F (0°C to 10°C) to trigger proper bulb development.
Greenhouse growers can achieve this chilling requirement through two main methods. One method is to maintain the entire greenhouse at the required low temperature during the initial establishment phase, which mimics a mild winter for the crop. A second, more controlled approach involves pre-chilling the individual cloves before planting them into the soil.
To pre-chill, cloves are separated from the bulb, placed in a breathable container like a mesh or paper bag, and stored in a refrigerator for 6 to 8 weeks at a temperature of 34°F to 40°F (1°C to 4°C). This technique is particularly useful for heated greenhouses or for varieties like hardneck garlic, which have a stricter cold requirement.
Soil Preparation and Planting Depth
Garlic requires a specific soil structure to allow for proper bulb expansion, making careful preparation of the growing medium essential. The soil must be loose, well-draining, and rich in organic matter to prevent the bulbs from rotting. A soil pH level between 6.0 and 7.0 is considered optimal for nutrient uptake and growth.
If planting directly into a greenhouse bed, incorporating well-rotted manure or compost several weeks before planting improves fertility and drainage. For container growing, which is common in greenhouses, using a high-quality potting mix with added grit or perlite ensures the necessary drainage, as garlic dislikes perpetually wet roots.
When planting, each clove should be placed with its pointy side facing up. The ideal planting depth is about 1 to 2 inches below the soil surface. Cloves should be spaced approximately 4 to 6 inches apart to allow sufficient room for the bulb to swell without crowding.
Managing Light, Water, and Curing
Once the cold period is complete and the shoots emerge, the plants enter the main growth phase, demanding consistent management of light and moisture. Garlic is a sun-loving plant, needing at least six hours of full sun daily, with 10 to 14 hours of total daylight being highly beneficial for maximum bulb size. In winter or short-day conditions, supplemental lighting may be necessary to ensure the plants receive adequate energy for photosynthesis.
Watering must be consistent to keep the soil evenly moist but never saturated, as waterlogged conditions quickly lead to fungal diseases and bulb rot. Watering at the base of the plant, often using drip irrigation, is preferable to overhead watering, which can encourage leaf wetness and disease. During the main growth period, garlic benefits from a nitrogen-rich fertilizer application to support leaf growth.
As the bulbs mature, usually when the lower leaves begin to turn brown and dry out, watering should be reduced or stopped completely for the final week or two before harvest. Garlic is ready to harvest when roughly half of the leaves have turned yellow or brown, but several green leaves remain.
After the bulbs are gently lifted from the soil, they must be cured in a warm, dry area with good airflow for two to four weeks. Curing is complete when the outer wrappers are papery and dry.