When should you plant garlic? The common advice is to plant after the first hard frost. That advice may work but it is not right. The best planting time has nothing to do with frost!
As garlic (Allium sativum) matures in the warm days of summer it enters a state of dormancy where it will not form roots or a shoot even if planted. This period lasts for 4 to 6 weeks depending on conditions and cultivar. If stored at room temperature the garlic slowly comes out of this rest period and is able to grow. The longer you wait, the higher the percent of germination. This process can be accelerated by exposing the seed garlic (i.e. cloves) to lower temperatures, a process called vernalization.
Many bulbs have a similar growth habit. For example, tulips will not grow unless they get a cold treatment. In cold climates we just put them in the ground to meet this need. In warm climates they can be put in a fridge for several weeks.
Planting Garlic Early
I did an experiment a few years ago where I planted garlic on Aug 2, Sept 1 and Oct 1, in zone 5. Our usual planting time is late October. All three plantings took place before a hard frost and they all performed well and there was very little difference in the harvest.
Clearly, waiting for the first frost is not a requirement.
Extension Office Recommendations
Most US extension offices have recommendations for starting garlic and South Dakota is typical, “10°C (50°F) at four-inches deep is ideal“. There is no requirement for a hard frost.
Planting Garlic in Warm Climates
Garlic is a popular crop in warmer climates where you can get two harvests a year, if the bulbs are provided with an artificial cold treatment. To meet this requirement scientists have determined the ideal vernalization conditions. A treatment of 5°C (41°F) for 20 days had the greatest yield. Another study used a cold temperature of 7°C (45°F) with storage periods of 0 to 30 days and found the longer storage period resulted in faster germination as well as a higher percent germination.
I found quite a few such studies and only one of them tested temperatures below 4°C (39°). Clearly they don’t think a frost is required.
Fungus Infection in Garlic
Fungus can be a serious disease in garlic. Botrytis sp, produce bulb and neck rot which affects onion, leek, shallots and garlic. The greatest incidence of infection occurs in persistent cool 10° to 24°C (50° to 75°F) and moist weather. Reducing temperatures below 4°C (40°F) reduces fungal infection and growth.
If you do have a problem with fungal infection it is a good idea to plant later when soil is cooler, but again frost is not a requirement. You can also disinfect your garlic before planting.
Concern Over Fall Growth
One reason gardeners suggest planting after a hard frost is that planting later results in less leaf growth above soil level, which is perceived to be a positive. This growth indicates that the bulb has set down a good root system and has started making leaves for spring. This is critical for an early spring start and the production of large bulbs next summer.
Leaf growth normally stays below ground but if late fall is warm it can show above the ground. Such growth is quite hardy and can tolerate temperatures down to -7°C (20°F). If you see such growth, cover with mulch and it will be fine over winter, even in zone 5.
Really Early Planting
What happens if you plant really early? As I was planting my garlic, I found some smaller bulbs that had been in the ground for at least a full year and maybe two. They are the result of letting one plant go to seed, which produces baby garlic bulbs. These fell to the ground and I just ignored them. Nobody even buried them.
I dug them up at the end of September and they were already producing both green shoots and lots of roots. Garlic left to their own devices starts growing much earlier than the first hard frost.
Is Hard Frost Needed?
Common gardening advice suggests that you should plant after a hard frost. The science says frost has nothing to do with the planting time. In colder climates you can plant much earlier and get a good harvest. In warm climates a vernalization period in the fridge should allow you to plant any time.