When is the Best Time to Plant Garlic?

T. Cranmer – Vegetable Crop Specialist
S. Vickers – Soil Management Specialist – Horticulture

How early is too early to plant garlic? Why was October the most popular time to plant hardneck garlic in Ontario? In 2022 we set out to determine when the best time to plant garlic is and over a three year trial, the results seem to indicate that the later garlic is planted, the greater the amount of risk there is to losing yields at harvest the following July.

Fall root establishment before winter is important to anchor the clove and protect against frost heaving that can result in winterkill. Planting around the date of the first fall frost seems to give plants enough time to establish roots, but not enough time to send up above ground leaves known as ‘bolting’ (Figure 1). When cloves are planted over a month before the fall frost, there is a good chance that there will be several leaves that emerge prior to the deep freeze of winter. Bolting is why many choose not to plant early, fearing that the plant is using up valuable energy in growing leaves that may not survive the winter.

To determine if planting earlier would result in winterkill or have negative impacts on yield, a planting date trial was set up in Dashwood, ON in 2022 with seven planting dates starting August 30^th and continuing roughly every three weeks until December 6^th, along with a single planting date in April 2023. Cloves were planted on similar dates in 2023–2024 and again in 2024–2025 (Figure 2).

In the first two years, the earliest planting date had best winter survival, the most leaves emerged prior to snow cover / the ground freezing and the best yields (Figures  3 & 4). In the last year of the study, the earliest planting date was moved from August 30th up to August 21, and for the first time in the study, we found a slight but not-significant decrease in plant survival and yield (Figures 3 & 4).  

The average date of the first fall frost in Dashwood is October 13^th based on the Average Frost-Free Period in Climatic Zones of Ontario publication (1975–2005). Planting around August 30^th would be almost six weeks before what was considered to be the ‘ideal’ time to plant. Planting in early September gives a much larger window of opportunity to get the crop planted, especially in heavier soils that take more time to dry out.

In addition to planting garlic at various planting dates, we also intercropped various cover crops in the fall and spring in the last two years of the trial to determine if a cover crop could be used to provide weed suppression and/or reduce soil erosion. In the 2023–2024 trial, two seeding dates of oats were planted alongside the garlic August 30^th, and crimson clover was planted the following spring on March 28^th. With a relatively low amount of weed pressure in this trial, it was found that garlic cloves could be intercropped/overseeded with a fall-seeded oat cover crop to provide good weed suppression, considerable soil erosion reduction, and relatively small yield drag (Table 1 of 2023–2024 planting date trial)

In 2024–2025 oats, sorghum Sudan grass and crimson clover were also sown on August 21, immediately after the garlic was planted. With the presence of grassy weeds in the field, it greatly reduced yields in the fall-seeded oat and Sorghum Sudan plots indicating that these cover crops may only be suitable in fields that have relatively good weed control to begin with, and where broadleaf weeds are the primary concern. (Table 1 of 2024–2025 planting date trial)

For those who have planted garlic earlier in the past and experienced winterkill, was the planting stock compromised or weakened in any way? Were there stem and bulb nematodes present? Were the bulbs cracked greater than seven days prior to planting? Consider planting 100 cloves in early September, or even August, and see how they compare to the rest of the field. 

All three of the planting date trials can be downloaded from the links below.