How to Grow Sunflowers

Pick the type before you buy seed

Sunflowers sold as “sunflower seed” range from 2-foot patio plants to 12-foot giants, and the type you choose changes how you space, feed and harvest them. Decide what you want the plants for first.

Single-stem cut-flower types make one bloom per plant on a straight stem. Many are pollenless, which means no yellow dust on the table and a longer vase life; NDSU Extension reports pollenless stems lasting up to two weeks in plain water. They still produce nectar, so bees and butterflies visit them. The ProCut series is the standard here: NDSU Extension recommends it for North Dakota, and Utah State University Extension says ProCut varieties have been highly successful on Utah flower farms. Utah State also lists ProCut White Lite, ProCut White Nite and Sunrich Summer Provence. NDSU notes the Sunrich series blooms about 10 days after ProCut, which helps stagger a harvest.

Branching types produce several smaller flowers per plant over a longer period, so you sow fewer times. The best branching varieties in NDSU trials were Shock-O-Lat, Moulin Rouge (also sold as Rouge Royale), Buttercream, Gold Rush and Strawberry Blonde. Utah State lists Gold Rush, Chocolate and Strawberry Blonde, but points out that branching stems are shorter (about 10 to 18 inches) and shed pollen, which shortens vase life.

Giant and seed types are what most people picture. Colorado State University Extension names ‘Mammoth Russian’ and ‘Russian Giant’ as popular giants in Colorado, and ‘Sunspot’ as a 2-foot plant carrying heads up to 10 inches across. For eating, grow confection sunflowers, which have large striped seeds; UGA Extension recommends black oil types such as Black Peredovik if you are growing seed for birds and wildlife.

In Florida, UF/IFAS recommends the cultivar ‘Valentine’.

When to plant

Sunflowers are a warm-season annual, but sources disagree on how warm the soil needs to be. Clemson Extension says to sow once frost danger has passed and the soil is above 50°F. UGA Extension prefers waiting several weeks past the last frost, until the soil reaches at least 60°F; in Georgia that means March in the south of the state and early May in the north. Utah State, working in a dry, cold-spring climate, sows once soil reaches 45°F and starts its first planting 7 to 14 days before the last frost, because seedlings survived down to 28°F in its trials.

The practical reading: in cool, dry regions an early sowing is a reasonable gamble, while in the humid South, UGA’s advice to wait several weeks costs little in a season that long. The dates for your zone above start at the last frost and run a few weeks past it.

Sunflowers are not a one-time crop. A single-stem plant gives one flower, so sow a new short row every two weeks (UC Master Gardeners say every 2 to 3 weeks; Utah State growers sow every 1 to 3 weeks). Stop when there is no longer time for the last sowing to bloom: Mississippi State Extension counts back 60 days from the first fall frost, and Utah State’s last sowing in northern Utah falls 7 to 14 weeks before the first hard frost.

Sowing: depth and spacing

Sources recommend different depths, and soil explains most of the difference. NDSU and Utah State sow 1/2 inch deep. UGA plants 1 to 2 inches deep and goes deeper in sandier soil, where the surface dries out fast. In heavy clay, stay near 1/2 inch; in sand, an inch or a little more keeps the seed in moist soil.

Spacing controls flower size, so set it on purpose:

• Small bouquet blooms (2 to 3 inches): Utah State found that 2 to 4 inches between plants produced blooms this size.
• Standard cut flowers (4 to 5 inches): 4 to 6 inches apart in the row (Utah State), or 6 by 6 inches in a bed (NDSU).
• Larger blooms and thicker stalks: NDSU thins single-stem types to 6 to 12 inches and uses 12 by 12 inches in beds for big blooms. UGA sows 6 inches apart and thins to 12 to 15 inches, with rows 2 to 3 feet apart.
• Branching types: about 24 inches apart (NDSU) or 18 to 24 inches (Utah State).
• Giants: Penn State lists 12 to 24 inches for giant common sunflowers.

Sow a little thicker than you need and thin once seedlings are up, because not every seed comes up and birds take some. Seedlings take about 11 days to appear, according to the Sacramento County Master Gardeners (UC ANR).

Starting sunflowers in trays

Direct sowing is standard because sunflowers root fast and dislike root disturbance. The exception is a garden with heavy bird or rodent pressure. A Mississippi State Extension agent sows in 72-cell trays, sets the seedlings out at 8 to 10 days old, 6 inches apart, and gets flowers about 50 days later. Moving them that young keeps root disturbance to a minimum, which Utah State stresses for transplants. Utah State sows two seeds per cell, 2 to 3 weeks before transplanting, with emergence in 7 to 14 days at 70°F. Do not hold seedlings in trays until they are root-bound; Utah State advises setting out plugs with well-developed roots that have not circled the cell.

Watering

Water lightly and often for the first 7 to 10 days so the top inch of soil never dries while seed is germinating. After that, switch to deep, infrequent watering. UGA cuts back to a couple of times a week without rain; UC Master Gardeners water once a week with several gallons; Utah State gives about 1 inch per week, once a week, after establishment. The point of deep, infrequent watering is deep roots, which UC Master Gardeners give as the reason for the schedule; once established, Utah State describes sunflowers as quite drought tolerant.

Overwatering is the more common mistake. Utah State warns that waterlogged soil leads to root and stem rot, which shows as dull, wilting foliage, then yellowing and stunting, with dark, soft roots. Sunflowers grow in almost any soil that is not constantly wet.

Feeding

Sources disagree here too. UC Master Gardeners say sunflowers on good soil need no fertilizer. Clemson says branching and large-flowered types can be heavy feeders and suggests composted manure plus regular slow-release fertilizer. Utah State puts their needs at low to moderate. UGA takes a middle line: without a soil test, apply 25 pounds of a balanced fertilizer such as 13-13-13 per 1,000 square feet (about 2 1/2 pounds per 100 square feet) before planting, then side-dress with 5 pounds of 34-0-0 per 100 feet of row and water it in. A soil test is the best guide, and Utah State adds phosphorus and potassium only when a test shows they are needed, since both build up in soil.

Soil pH matters less than drainage. UGA recommends 6.0 to 6.8 and Clemson 6.0 to 7.5.

Staking, pinching, mulch and weeds

UC Master Gardeners recommend staking any sunflower over 3 feet tall and all multi-branched plants. Clemson adds that large cultivars may need stakes because of strong winds or heavy seed heads. Cut-flower rows at close spacing usually hold each other up, although Utah State notes that crowding also raises the risk of plants falling over in windy regions.

Pinch branching types, never single-stem types. NDSU pinches branching plants at about 16 inches tall to get longer side stems for cutting; Utah State pinches just above the second set of true leaves. Pinching a single-stem variety can leave you with poor blooms or none.

Once plants are growing well, spread 2 to 3 inches of mulch, kept back from the stems. Weeds are the main early competitor for water and nutrients, so hoe or pull them while the sunflowers are young.

Regional differences

Cold north and mountain West. The season is short, so fast varieties matter. NDSU sows after the last frost, which is mid-May for most of North Dakota. ProCut types blooming in 50 to 60 days leave room for several successions. In Utah’s dry springs, growers sow a week or two before the last frost because seedlings tolerate a light freeze.

Humid South and Gulf Coast. Wait for 60°F soil. Mildews and rust can show up on older leaves, but UGA notes that vigorous plants usually grow through them. The long season allows sowings from spring through late summer; one Mississippi State agent on the zone 7b/8a line harvested weekly for about 10 weeks until November 1.

Arid West and low desert. Sunflowers are well suited to dry heat. The University of Arizona’s low-desert guide lists planting from February through mid-July with blooms May to October. Use drip or water at the base; Utah State does not recommend overhead watering for cut flowers.

Florida and warm-winter areas (zones 9 to 11). UF/IFAS says plants sown from late winter to early spring bloom by mid-spring and decline quickly once late-spring heat arrives. Its monthly calendar lists sunflowers for March, April and June across the state. Fall plantings give a shorter display of several weeks.

Cutting sunflowers for the vase

Timing makes the biggest difference to vase life. NDSU says to cut when the flower shows the first signs of opening. Utah State describes the window as the “lift” or “cup” stage, when the petals have begun to rise off the center disk but have not opened flat. Once the center looks fuzzy, the flower is past its best. In Utah State’s figures, stems cut at the right stage held 7 to 10 days in a cooler at 36 to 41°F, while stems cut later held only 3 to 5 days.

Cut in the early morning or cool evening; UC Master Gardeners note that stems cut in afternoon heat tend to wilt. Strip all leaves except the two nearest the bloom and put the stems straight into clean, cool water. On branching plants, cut each stem at the base of its own branch so the buds below keep coming.

Harvesting sunflower seed

Seed heads take 80 to 120 days from sowing, and you check them in early fall. Look for three signs: the back of the head turns from green to yellow, then brown; the head droops; and the small florets covering the seeds dry and fall out, exposing plump, tightly packed seed.

Birds and squirrels will usually find the seed first. Cover heads with paper bags or cloth as they start to droop, or try scare devices such as spinning pie pans or an owl decoy. To harvest, cut each head with a foot or more of stalk attached and hang it in a warm, dry, ventilated spot away from rodents and humidity. After several weeks of curing, rub two heads together or brush the seed out by hand. UC Master Gardeners let the loose seed dry for another 1 to 2 months, then store it in airtight glass jars in the refrigerator.

Problems and fixes

• Seed or seedlings vanish: birds and mice. Set out tray-grown seedlings instead of sowing seed.
• Young plants eaten to the ground: deer can destroy a planting quickly. UGA advises repellents or fencing where deer are common.
• White or gray coating on lower leaves: powdery or downy mildew, which usually starts on the oldest leaves. Space plants for air movement. Healthy plants usually outgrow it, and late-season mildew rarely needs treatment. NDSU notes that breeders now offer varieties resistant to downy mildew.
• Rust on leaves: another fungal disease of older foliage; like the mildews, it seldom stops a vigorous plant.
• Bugs on stems and heads: leaf-footed bugs, stink bugs and aphids. Sunflowers draw so many beneficial insects that UGA advises thinking carefully before spraying. Wash aphids off with a strong stream of water.
• Tunnels or damage in the head: the sunflower moth lays eggs in developing flower heads, and caterpillars feed inside.
• Plants wilt and yellow in wet soil: root rot from waterlogging. Improve drainage and water less often.

Some gardeners plant sunflowers to lure stink bugs and leaf-footed bugs away from vegetables; UGA mentions this trap-crop use. The UC Master Gardener advice to keep sunflowers away from potatoes and pole beans, because they release substances that slow other plants, is long-standing gardening guidance rather than a trial result.

Common mistakes

• Sowing by the calendar alone. Check soil temperature: at least 50°F (Clemson), and 60°F in the South (UGA).
• Sowing everything at once. Single-stem types bloom together and finish together. Sow every two weeks instead.
• Crowding giants. Giant varieties need 12 to 24 inches; at cut-flower spacing they never get big.
• Watering a little every day. Shallow roots make floppy plants. Water deeply and less often once plants are up.
• Cutting fully open flowers. Utah State found late-cut stems kept only 3 to 5 days in a cooler. Cut at the cup stage.
• Leaving seed heads unprotected. By the time the back turns brown, birds may have stripped the head. Bag it as soon as it droops.