May is generally the best time of year to plant sunflowers in most parts of the United States. In general, it is safest to plant sunflower seeds after the last frost date in your area, but they can be planted up to 4 weeks before the last frost date. Just be sure to protect the seedlings with a cover if a frost is forecasted.
The best time to plant sunflower seeds is generally during a three-month period starting about one month before the local last frost date. This typically means planting in mild coastal climates in March, April, or May. In cold interior climates, the ideal months for planting are May and early June.
Sunflowers are typically planted outdoors when soil is between 50°F (10°C) and 86°F (30°C). While most varieties will germinate at a wide range of temperatures (between 41°F (5°C) and 100°F and (38°C)), the optimal soil temperature for sunflower seed germination is about 77°F (25°C).
When to plant sunflower seeds
Sunflower seeds are most commonly planted in April or May. Planting in late April or early May when the soil is warm (above 50°F/10°C) gives seeds enough time to germinate and grow before summer’s hot temperatures arrive. If temperatures are not optimal, the germination process is delayed, leaving the seed exposed to potential soil-borne diseases and insect pests.
Because sunflower seedlings are somewhat frost tolerant, you can plant the seeds about a month before the average last frost date if you wish. This is particularly common for gardeners and farmers who plant a new batch of sunflower seeds every two weeks over several months. That way, if the first planting gets damaged by a cold snap, there are still lots of lovely flowers left to bloom.
Best months for planting sunflower seeds
Sunflowers can usually be planted about a month before the local last frost date. In a coastal climate with a late March frost, sunflower seeds can be planted outdoors starting in March and replanted at two-week intervals through April and May. In colder prairie climates with mid-May frosts, sunflower seeds are usually planted in the ground in May or June. If planting in June in a cooler area, choose a cultivar with a short days-to-maturity.
Soil temperature for sunflower seed germination
Sunflower seeds can germinate in the soil as cold as 4°-6°C, although they will take much longer to do so than if they were at a warmer temperature. Some varieties germinate better at a low temperature than others. It’s also important to note that the soil needs to be a minimum of about 40°F (5°C) at the depth of the seed (usually about an inch or two).
Planting sunflower seeds into cold soils (below about 50°F/10°C) can delay germination, so be wary when planting in cooler climates. The soil may be more prone to fungal disease, and the weakened seedling may be more likely to suffer from dampening off. For this reason, most gardeners wait for the soil to warm to over about 50°F/10°C before planting sunflower seeds.
Sunflower plant lifespan and timing
Plant the seeds promptly so the sunflower heads can reach full maturity before the wet season in the autumn.
When selecting a planting date, several factors must be considered beyond soil temperature, such as how often you plan to water them, predicted precipitation levels, length of the local growing season, and forecasted temperature changes during germination.
Time for sunflower seeds to germinate
Temperature is the principal variable that controls the germination rate of seeds. Sunflower seeds usually germinate within a week when the soil temperature is optimal (around 77°F/25°C). Germination may take two or more weeks at sub-optimal temperatures.
Seeds do not germinate at all below a certain minimum temperature (base) or above a certain maximum temperature (ceiling). The minimum temperature for sunflower seed germination is about 41°F (5°C), while the maximum is up around 100°F (38°C).
Expected blooming season of different sunflower varieties
Here are the expected days-to-bloom required after direct seeding in the outdoor garden for some common sunflower varieties:
- Big Smile sunflower: 50-60 days
- ProCut Orange sunflower: 50-60 days
- Autumn Beauty sunflower: 75-85 days
- Mammoth sunflower: 75-90 days
- Soraya sunflower: 85-95 days
Replanting sunflower seeds
While very young sunflower seedlings are somewhat frost-tolerant, the plants lose their tolerance to freezing temperatures once they start to grow true leaves. Young seedlings can be damaged by cold temperatures in the spring or by other environmental factors like lack of water, the presence of pests, or weather events like wind or hail. If this happens, you can usually replant your crop so you can still harvest sunflowers in the fall.
“Sunflowers are fairly frost tolerant up to the V4 or 4 leaf stage. The cotyledons just emerging are most frost tolerant, with risk of injury increasing as the plant adds leaves. If sunflowers become brown or black and the terminal bud is severely damaged, plants will not recover. Less severe frost damage may result in loss of apical dominance and plants will exhibit branching from axillary buds, resulting in multiple heads later in the season.”
https://www.gov.mb.ca/agriculture/crops/crop-management/print,spring-frost-damage-bulletin.html#sunflowers
“Base temperatures obtained in this study ranged between 3.3°C and 6.7°C whereas maximum temperatures ranged between 41.7°C and 48.9°C.”
https://www.isasunflower.org/fileadmin/documents/HELIA_issues/Helia33/Helia_33__11_pp_97-104_.pdf
