Sunflowers, especially smaller varieties, make a wonderful garden flower, even if planted in a small area. They are bright, beautiful, and hardy and will attract birds and bees. They are also a popular commercial crop grown for their seeds and their oil, and the only one native to the U.S.
So, sunflowers are native to North America, but when is the best time to plant the seeds? Soil temperature is the vital factor here. Horticulturists advise that it’s important to wait until the soil has warmed to at least 50°F (10°C). It can be warmer than this, but it shouldn’t be colder.
Planting Sunflowers in Different Climates
All sunflower varieties need full sun at least for most of the day. So, it stands to reason that they will grow best in warmer climatic conditions. They will also tolerate drought.
You’ll be safe if you live in any of the USDA plant hardiness zones from 4 through 8.
But because they grow quickly, gardeners in Zones 2 and 3, and even up to Zone 11 also have success growing sunflowers.
Wherever you are, it’s essential to sow sunflower seeds only once the risk of frost has passed. A good rule of thumb is to plant about three weeks after the last spring frost.
Let’s look at the five different climate types to see how your climate will affect when you can start planting sunflower seeds.
All sunflower varieties thrive in the tropics and sub-tropics, as long as it isn’t too humid.
It is hot and humid in the tropics, with average temperatures exceeding 64°F (18°C) throughout the year. There are more than 59 inches (1.5 m) of rainfall every year, and no frost. It isn’t as humid in sub-tropical areas.
According to scientists, the temperature of the soil will be 35-39°F (2-4°C) warmer than mean annual air temperatures. So, it’s never going to be too cold to sow sunflower seeds.
A large part of the western U.S. has a dry climate. This is because there is very little rain, and moisture in the air evaporates quickly.
Sunflowers will usually do well in a dry climate. Conversely, they don’t do well in any area, regardless of climate, where the soil becomes waterlogged.
For planting, be guided by the temperature of the soil. Water regularly when the weather is very dry, especially while you wait for the seeds to germinate.
Sunflowers are widely grown in temperate regions, both commercially and in home gardens.
Temperate climate zones are typically warm and humid in summer and cool in winter. And much of the U.S., particularly in the east, has a temperate climate.
A fairly substantial part of the northeastern U.S. and some parts of the western country, experience a continental climate. In this climate zone, you can expect summers to be warm to cool and winters to be cold.
But temperatures can fall below -22°F (-30°C). This means that the growing season for sunflowers will be relatively short.
It gets extremely cold in the polar regions. Even in the summer months, the temperature never rises above 50°F (10°C). So, it’s not a good idea to plant sunflowers at all.
Choosing Sunflower Seeds
There are two basic types of sunflower, the annual plant Helianthus annuus, and the lesser-known, easy to grow perennial plants that keep on reseeding every year. But there are many Helianthus species in both groups.
Almost all the commercial varieties of Helianthus annuus are hybrids, and most seed catalogs recommend sunflower varieties that will grow best in different regions. The choice is mind-boggling and includes the full range of sizes from dwarf to giant, all with glorious flower heads for cutting flowers you can display in a vase.
For example, you can buy:
- Multi-branching half-hardy annuals that are ideal for borders and pots
- Hardy dwarf annuals that can be used to create an unusual hedge
- Hardy annuals with bright lemon-yellow, cream, red, or bi-colored flowers
The most common perennial plant, Helianthus laetiflorus, grows wild, often alongside roads.
It is also available commercially.
Helianthus doronicoides is another perennial sunflower from the Asteraceae family. It is a hermaphrodite, with both male and female organs and is pollinated by bees and flies. It can be planted in forest settings.
The whorled sunflower, Helianthus verticillatus is listed as an endangered species in the U.S. They grow naturally in woodlands and prairie-type environments but are threatened by pine plantations and industrial forestry.
Unless you have your mindset on a specific sunflower species, have a look at seed catalogs and the seeds available in your local garden centers. Decide on the effect you want to create with your sunflowers in terms of color and size.
Some sunflowers grow up to 12 feet tall while others only reach 4 feet. The size of the sunflower head also differs depending on the variety you plant.
If you want to grow sunflowers in pots, then be sure to choose a small variety. Also, be sure that the pot you choose has enough depth to accommodate the taproot of the plant.
How to Plant Sunflower Seeds
Sunflowers are very easy to grow from seed. The first step before you plant the seeds is to make sure that the soil in your garden beds is suitable. Even though sunflowers aren’t particularly picky about soil quality, they will grow better in rich soil.
According to the University of Georgia Extension, when you grow sunflowers in a home garden it is advisable to do a soil test to assess the nutritional needs of your soil. They advise that they will do best if the soil is slightly acidic, with a pH between 6.0 and 6.8.
If you don’t (or can’t) do a soil test, use a balanced fertilizer to improve the quality of the soil. Dig this into your garden bed.
The sandier the soil, the deeper you will need to plant the seeds. So, if it is sandy, add lots of organic matter.
Planting Sunflower Seeds
If you are going to plant in rows, make sure there are at least 2-3 feet (60-90 cm) between the rows. Then plant the seeds about 6 inches (152 mm) apart and between half and one inch deep, depending on the variety.
To extend the growing season of your sunflowers, plant seeds every 7-10 days. It will usually take between 10 and 14 days for each batch to sprout.
Once the seedlings emerge, presuming they all grow, thin them out so that there are 12 inches between the plants.
It takes between 50 and 100 days for sunflowers to bloom after planting sunflower seeds.
How to Water Sunflowers
It takes 5-10 days for sunflower seeds to germinate. Because the seeds contain a lot of natural oil, they need a fair amount of water for germination.
To get germination going, water the bed where you have planted the seeds lighting for about 7-10 days.
As they grow, continue to water the plants regularly to ensure that the soil stays moist, but not wet. They usually need about an inch of water once a week throughout the growing season.
A good rule of thumb is to spray with water and check that the top 6 inches (15 cm) are moist. This might be more of a challenge in dry climates. But don’t panic because they can withstand heat and even drought once they are established.
How to Grow Sunflowers
Once your sunflowers are established all you need to do is let each plant grow happily in full sun. According to the Texas A&M AgriLife Extension, they will survive even if the weather gets cold, but not below 26°F (-3°C).
One fascinating aspect of growing sunflowers is that they are heliotropic. The young flower heads and leaves face east in the morning but follow the movement of the sun so that they face west in the evening.
Heliotropism decreases gradually during flowering with most mature heads eventually facing east. At this stage, the flower heads of many varieties tend to face the ground. This is nature’s way of stopping birds from eating the seeds.
How Long Do Sunflowers Take to Grow?
All sunflower varieties grow very quickly. Some of the giant species can grow 12 feet tall in only three months.
Although the different varieties grow at different rates, in general, they need between 70 and 100 days to reach maturity. Most annuals will bloom throughout the summer months and will die back in the fall, usually after the first frost.
Sunflowers are easy to grow. And as long as you plant the seeds once the danger of frost has passed, there is no reason why you won’t be rewarded with a sunny display in summer and the fall.
Even though they do best in warm climates, the beauty of sunflowers is that once they are established, they withstand cold temperatures and heat.
Wherever you are, why not find a field or just a corner to grow some sunflowers. They’ll be sure to put a smile on your face.