Garlic (Allium sativum) is a unique crop whose successful cultivation depends on meeting precise temperature demands throughout its life cycle. Unlike many vegetables, garlic requires distinct thermal signals to transition from a single planted clove to a fully segmented bulb. The temperature dictates whether it will produce only leaves or properly divide into marketable cloves. Understanding these thermal needs is foundational for a successful harvest, as the journey involves sequential temperature phases, each serving a specific biological function.
The Critical Cold Requirement for Bulb Formation
The most significant temperature requirement for garlic is vernalization, the necessary exposure to cold temperatures that triggers the plant to develop a segmented bulb. Garlic planted in the fall must undergo this chilling period to switch from a vegetative state (producing only leaves) to a reproductive state where the bulb divides into cloves. Without this cold exposure, the plant will fail to form a full bulb, instead producing a single, undivided sphere known as a “round.”
The optimal temperature range for vernalization falls between 32°F and 50°F (0°C and 10°C). This cold treatment must be sustained for six to eight weeks, though some varieties require a minimum of 40 days below 40°F (4.5°C).
In regions with mild winters, growers often simulate this chill by artificially vernalizing seed cloves in a refrigerator before planting. This ensures the physiological changes necessary for bulb formation are met. If the cold duration is too short or the temperature is too high, the plant will not receive the necessary signal, resulting in a reduced and often unmarketable harvest.
Optimal Temperatures During Active Spring Growth
Following the vernalization period, the garlic plant enters a phase of rapid vegetative growth, focusing on producing foliage and establishing a robust root system. During this stage, the plant performs best in moderate air temperatures, ideally ranging between 50°F and 75°F (10°C and 24°C). These conditions support the vigorous photosynthesis needed to store energy for the eventual bulb swelling.
Soil temperature plays a large role in root establishment and nutrient uptake, with the plant thriving as the soil begins to warm up. The goal during this time is to maximize the plant’s size above ground, as the final bulb size is directly related to the amount of foliage produced before bulbing begins.
The plant is sensitive to the length of the day during this spring phase; the combination of warming temperatures and increasing daylight hours triggers the plant to cease leaf production and begin the final bulbing process. Maintaining moderate conditions without excessive heat stress allows the plant to accumulate the necessary resources. The plant can tolerate spring cold snaps, but sustained temperatures above the ideal range can prematurely halt vegetative growth, leading to smaller bulbs.
Heat Requirements for Maturation and Curing
The final stage of development requires a shift to warmer, drier conditions to prepare the bulb for harvest and storage. As summer approaches, temperatures above 80°F (27°C) combined with reduced soil moisture are beneficial for the final swelling and hardening of the bulb. This warmth encourages the plant to transfer stored energy from its leaves into the underground bulb, maximizing its size and density before the foliage dies back.
Once harvested, the bulbs must undergo curing, which requires specific elevated temperatures for long-term storage. Curing involves drying the neck and outer wrapper layers of the bulb, sealing the cloves and preventing moisture loss and disease entry.
Curing Temperature and Duration
The ideal environment for curing is a warm, well-ventilated space maintained between 75°F and 90°F (24°C and 32°C). This high-temperature, low-humidity environment must be sustained for two to four weeks. The warmth accelerates the drying process, which is necessary to achieve the dry, papery outer skin that ensures a long shelf life. Avoiding temperatures above 90°F (32°C) is important to prevent damage to the bulb’s structure.