When to Plant Sunflowers. It's best to sow sunflower seeds directly into the garden (or outdoor containers) after the danger of spring frost has passed anytime after soils have warmed to at least 50°F (10°C). In the northern half of the U.S. and in Canada, this will fall between April and mid-June.
So how late can you plant sunflower seeds? With varieties that only require a 50-60 day growing season, you should be able to plant sunflowers as late as July for fall flowers. If you live in a warm climate where the first frost comes well into late fall, you can plant sunflowers even later in the season.
You can still get a second blossoming of sunflowers as long as it isn't too cold though. In USDA zones 8 and higher you should be able to get in a second crop of sunflowers but watch out for early frosts. Start sowing the seeds in mid or late August for the best results.
Let me share the ins and outs of each of these three sunflower growing options. Sunflowers are easy to grow from seed planted in the early spring, mid spring, or even the winter using different techniques.
The answer is no. It's possible to sow sunflower seeds in summer, up until August and possibly even later, depending on when your fall frost begins.
Sunflowers can survive in extreme heat, but they grow best in temperatures between 70 to 78 degrees. Not only that, but sunflowers can tolerate periods of drought as long as they receive enough water while their roots are growing.
When to Plant Sunflowers. Plant seeds after the danger of spring frost has passed and the soil temperature is at least 60 degrees. This will be between March and May, depending on where you live.
Many flowers carry the name “sunflower” that are, in fact, completely different species of plants. Some sunflowers are annual and survive only one season, and others are perennials that will return every year. All perennial and annual sunflowers produce brilliant flowers that are a highlight of any late-summer garden.
Sunflowers grow quickly. Many can achieve up to 12 feet of growth in only 3 months. With the proper growing conditions, sunflowers should reach maturity in 70 to 100 days after planting.
The allelopathic characteristics of sunflowers are toxic to some plants, potatoes being one of them. This process inhibits germination and can stunt the growth of potatoes. They also compete for nutrients in the soil. Potatoes should be planted well away from sunflowers to avoid them affecting one another.
Place one seed in the divot and cover it with soil. If you have extra seeds, you can put 2-3 in one hole and then trim off the two weaker seedlings. Water the soil gently to help settle it. Sunflower seeds of most varieties should be planted about 18-24 inches apart, as they will grow to be quite large.
Temperature and Humidity
Optimal temperatures for growing sunflowers are between 70 and 78 F. Still, they tolerate high heat as long as their moisture needs are met. Sunflowers can handle somewhat chilly but sunny environments.
Sunflowers need full sun; see 6-8 hours of direct sunlight per day – the more the better if you are trying to grow them to their maximum potential. Choose a well-drained location, and prepare your soil by digging an area of about 2-3 feet in circumference to a depth of about 2 feet.
When planting sunflower seeds, tuck them into soil between 1 and 2 inches deep. Space them roughly 6 inches apart. Soil temperature and moisture influence how long it takes for sunflower seeds to germinate, but in most cases you should see sprouts in 7 to 10 days.
With proper care, sunflowers should last from six to twelve days. To maximize the vase life, look for flowers that are just starting to fully open.
Newly planted sunflowers need damp conditions, especially during germination, but established sunflower plants need about one inch of water per week. Underwatered sunflowers will not grow large or bloom, and the plants may droop and appear wilted. Overwatered sunflowers are susceptible to root rot.
How much can a sunflower grow in a week? A sunflower may grow between one to 12 inches per week, depending on the variety and growing conditions. If conditions are optimal, a sunflower that will be 6-feet tall at maturity in 85 days (12 weeks) would average 6-inches of growth a week.
Sunflowers are fast reproducers, and one plant can create dozens of others. Plant seeds about an 1 inch below the ground. Germination occurs quickly, usually between 5 and 10 days, but spring crops produce more plants than summer crops. Spring crops are planted in April and May.
When do sunflowers bloom? Annual sunflowers bloom during summer and into autumn. Sow new plants every few weeks and you'll enjoy non-stop flowers until the first frost. Perennial sunflowers bloom for a period of 8-12 weeks with some beginning as early as July and others finishing as late as October.
There are pint-sized sunflowers that grow just a foot tall and giant varieties that reach for the sky, but you don't need a big garden to grow sunflowers. These classic summer bloomers can be planted in plastic pots, fabric planters, or even buckets.
Keep them deadheaded until the end of the season. If you deadhead your sunflowers, they will keep pumping out new blossoms in their will to create seeds and more sunflowers. Don't cut the stalk way back, the next sunflower often forms just inches from the place you deadheaded.
You can still plant flowers, too!
Planting later crops in June also applies to flowers! Plant more marigolds, sunflowers, cosmos, zinnias, four o'clocks, nasturtiums, etc. – all of the annuals that usually burn out by late summer.
Deer will sometimes eat sunflowers, so be aware if you live in an area with large deer populations. Sunflowers are available as annuals and herbaceous perennials and are only susceptible to hungry animals during the spring, summer, and early fall.
Sunflower seedlings should be watered daily so the soil is moist but not soaked. As the sunflowers become established, they can be watered once a week.