When to Plant Sunflowers

Updated August 2026 · Garden Advice Guide

Direct sow sunflower seeds outdoors after your last spring frost, once the soil at 2 inches deep reaches 55 to 60°F. For most of the US that lands somewhere between April and June. Sunflowers grow so fast from seed and resent transplanting so strongly that direct sowing beats starting indoors in nearly every situation.

Your whole schedule keys off your last frost date, so look yours up with our free frost date lookup and then use the zone table below.

Arriving here in late August? Sowing season is over for most zones, but this is peak harvest season. Skip down to when to harvest sunflower seeds for the full process.

Why you should direct sow (and skip the seed trays)

Sunflowers grow a taproot. Within two weeks of germinating, a sunflower seedling has sent a single central root straight down, and that root is the reason mature plants stand up to wind and pull water from deep soil in August.

Lift that seedling out of a cell tray and you break the tip off the taproot. The plant survives, but it spends its first two weeks rebuilding instead of growing, it stays shorter, and it's more likely to flop in a storm later. Meanwhile a seed sown directly into 60°F soil is up in 7 to 10 days and grows fast enough that it catches the transplant within a month.

The math almost never favors transplanting. A sunflower goes from seed to bloom in 55 to 75 days for most varieties, which is short enough that even zone 3 gardeners can direct sow and still get flowers.

The narrow exceptions:

• Very short seasons (zone 3) growing a 100+ day giant variety, where you genuinely need a 3-week head start.
• Heavy bird and rodent pressure. Birds, squirrels, and mice dig up sown sunflower seed constantly. If you lose every sowing, start in 3-inch peat pots or soil blocks and set them out at the two-true-leaf stage.
• Slug country in a wet spring, for the same reason.

If you do start indoors, go only 2 to 3 weeks early, use a pot that goes in the ground intact, and transplant before the seedling is 4 inches tall. Never a plastic cell tray, and never a plant already showing a flower bud.

The soil temperature rule

Sunflower seed germinates once soil holds at 50°F, but 55 to 60°F is the number to wait for. That's where germination is fast and even instead of slow and patchy.

University of Minnesota Extension notes that sunflowers tolerate cooler soil than most warm-season annuals and will handle a light spring frost once established, which is why they can go in earlier than corn, beans, or squash. That tolerance is real, but it applies to the seedling, not the seed. Cold soil still costs you germination.

Take the reading at 2 inches deep, mid-morning, three days running. An inexpensive soil thermometer settles a question that guesswork gets wrong every spring.

When to plant sunflowers by zone

These windows assume direct sowing. Your own frost date always beats the table.

Zones 9 and 10 should avoid midsummer sowings. Sunflowers bloom poorly and burn out fast in sustained 95°F-plus heat. A spring run and a fall run works far better than fighting July.

The last-sowing-date math

Work backward from your first fall frost:

1. Start with your first fall frost date.
2. Subtract the variety's days to maturity.
3. Subtract 2 more weeks as a buffer, because shortening days in September slow everything down.

Zone 6 example: first frost October 15, growing a 65-day cut-flower type. October 15 minus 65 days is August 11. Minus a 2-week buffer is July 28. So late July is the honest cutoff for that variety in that garden.

Same garden, growing a 110-day Mammoth for seed: October 15 minus 110 days is June 27, minus 2 weeks is June 13. Giant seed types have a much earlier deadline than people expect, and that's the mistake behind a lot of half-formed seed heads in October.

Succession sowing: the trick that keeps sunflowers blooming all summer

A single sunflower sowing gives you one burst of bloom. Single-stem varieties in particular flower once and are done, and even branching types peak and fade over about 4 weeks.

Sow a new short row every 2 to 3 weeks from your first sowing date through early July and you get continuous blooms from midsummer straight through frost instead of a two-week show in August. This is the single most useful sunflower tactic most gardeners never try, and it costs nothing but a few extra seeds.

A practical zone 6 schedule looks like this:

• May 15: first sowing
• June 1: second sowing
• June 20: third sowing
• July 8: fourth sowing (last one for a 65-day variety)

Four sowings, roughly 8 to 10 weeks of continuous bloom. If you're growing for cut flowers, tighten the interval to 10 to 14 days, which is what commercial cut-flower growers do.

Two notes that make succession work better. Give each sowing its own short row rather than mixing them, so the tall ones don't shade the young ones. And shift to faster varieties as the season goes on, using your 55 to 65 day types for the last two sowings.

Sunflower types and what each one is for

Choosing the wrong type is the most common sunflower disappointment. Someone wants bouquets and plants a 12-foot Mammoth that drops pollen all over the table and has one flower per plant.

Mammoth types are seed plants, not flower plants. One head, one bloom, then a month of the head hanging over while seed fills. Beautiful, but plan for it.

Pollenless types are single stem. That's the trade: you get a clean bouquet flower with no pollen mess, but the plant produces one flower and stops. Pollenless varieties also produce far less for pollinators, so if you're planting for bees, grow at least some pollen-bearing types alongside.

Branching types are the best value for cut flowers. A single Italian White plant will hand you 8 to 12 usable stems over several weeks. Six plants can supply a household all summer.

Growing notes: sun, spacing, water, and staking

Full sun, no compromise. Six to eight hours minimum, and more is better. Sunflowers in part shade get leggy, lean hard, and produce small heads. Young plants are heliotropic (the buds track the sun across the sky), but once the flower opens the head sets in one direction, typically facing east.

Spacing by type:

Crowding giants is the classic error. A Mammoth at 12-inch spacing makes a spindly stalk with a dinner-plate head instead of a trunk with a platter.

Water more than you expect, early. Sunflowers have a reputation for being drought tolerant, and mature plants genuinely are, because that taproot goes down 4 feet or more. But the first 20 days are the opposite. A young sunflower with a shallow root system needs steady moisture to establish, and a dry spell at the seedling stage permanently caps the plant's size. Water deeply twice a week until plants are knee high, then back off to weekly deep soaks. Water at the base, not overhead, once heads form; wet flower heads invite head rot.

Stake anything over 5 feet in a windy spot. A 10-foot sunflower carrying a 12-inch seed head is a sail. Drive a sturdy 6-foot stake 12 inches deep when the plant is 3 feet tall (not after, so you don't spear the root) and tie loosely with soft twine at 2 to 3 points. A row of tall sunflowers along a fence can be tied to the fence instead.

Feed lightly. Sunflowers do fine in average soil. Heavy nitrogen gives you soft, tall, floppy stems and smaller heads. If you feed anything, use a balanced or slightly phosphorus-leaning fertilizer at planting and leave it there.

Allelopathy: where not to plant sunflowers

Sunflowers are mildly allelopathic. Their roots, leaves, and especially their seed hulls release compounds that suppress germination and growth in some neighboring plants. This is a documented effect, not a garden myth, and it shows up in two places.

In the garden bed. Potatoes and pole beans are the two crops most often reported as struggling next to sunflowers. Give them a few feet of separation. Corn, squash, cucumbers, and most flowers do fine.

Under bird feeders. This is where the effect is unmistakable. Discarded black oil sunflower hulls pile up and build enough concentration in the soil that grass and seedlings simply won't grow in that patch. Rake hulls out regularly, or move the feeder each season.

The practical version is mild: don't compost large volumes of sunflower hulls into a seed-starting mix, don't plant potatoes at the base of a sunflower row, and don't worry much beyond that. Our companion planting chart has the full set of pairings worth respecting.

When are sunflower seeds harvested, and how to do it right

Late August through September is prime sunflower seed harvest across most of the country. Here's the full sequence.

Know when the head is ready

Look for all four of these together:

1. The back of the head turns from green to yellow, then to yellowish brown. This is the main signal. The front of the flower is misleading, so flip the head and look at the back.
2. The bracts (the pointed green leaf-like structures around the edge of the head) turn brown and dry.
3. Petals have dried and dropped, and the head hangs down.
4. The seeds are plump and full-sized, with clear black-and-white striping on striped varieties, and they loosen from the head with a rub of your thumb.

Timing-wise, seed heads are ready roughly 30 to 45 days after the petals drop. Iowa State University Extension advises harvesting when the back of the head has turned brown and seeds are fully developed, then finishing the drying indoors, which is exactly right in a climate with fall rain.

Pull one seed and bite it. A mature seed has a firm, filled kernel inside. A flat, papery shell means the head isn't done, or that it was never pollinated well.

Protect the head from birds and squirrels

The moment seeds fill, so does every bird and squirrel within a quarter mile, and they will strip a head in two days. Squirrels are just as determined about chewing into fall pumpkins, and the barrier logic is the same.

• Mesh or netting bags, or cheesecloth, tied around the head. Best option, because air still moves through.
• Paper bags (not plastic) with a few ventilation holes punched, secured around the stem with twine. Fine in dry weather; a soaked paper bag causes mold, so check after rain.
• Nylon stockings stretched over the head work well and let the head keep drying.

Put the covering on when the back of the head starts to lighten, not before. Covering a still-green head restricts airflow with no benefit.

Cut and cure

Cut the stem about 12 inches below the head with loppers or a strong pair of pruners. That stem is your handle and your hanging hook.

Hang the heads upside down in a dry, warm, airy place with good circulation: a garage, a shed, a covered porch, an attic. Not a damp basement. Space heads apart so air moves around each one. Curing takes 1 to 3 weeks depending on humidity, and it's done when the back of the head is hard and brown and seeds fall out with a light rub.

A bucket, a paper bag, or a sheet under the hanging heads catches the seeds that drop on their own.

Thresh, clean, and store

Rub two heads face to face, or run your gloved hand or a stiff brush across the face of the head, and the seeds release. Winnow the chaff by pouring the seeds between two bowls in front of a fan.

Then dry them properly before storage. Even cured seeds carry moisture. Spread them one layer deep on a screen or baking sheet for a few more days in a dry room, or in an oven set to its lowest temperature with the door cracked for a couple of hours. Seeds that go into a jar even slightly damp will mold.

Store dried seeds in an airtight jar or container in a cool, dark place. They keep about a year at room temperature, longer refrigerated, and 2 to 3 years frozen. Save your biggest, fullest seeds from your best plant for next year's sowing and label them with the variety and year.

To roast for eating: soak the seeds overnight in salted water (about 1/4 cup salt per 2 quarts), drain, spread on a baking sheet, and roast at 300°F for 30 to 40 minutes, stirring a few times, until they're crisp and lightly browned.

Or leave them standing for the birds

You don't have to harvest at all. A standing sunflower head is one of the best natural winter bird feeders there is, and goldfinches, chickadees, nuthatches, and cardinals will work it for weeks. Leave the stalks up through fall and winter, cut them down in early spring, and you get bird activity all winter plus hollow stems that overwintering native bees can use.

A good compromise: harvest half your heads and leave half standing.

Worked example: a zone 6 gardener sowing 'Mammoth' on May 20

Full season with the arithmetic shown.

The setup. Zone 6, last frost May 10, first fall frost October 15. Variety: Mammoth Grey Stripe, listed at 110 days to mature seed. Goal is edible seed, not cut flowers.

May 20: sowing. Soil thermometer reads 62°F at 2 inches. Seeds sown 1 inch deep, 30 inches apart, in a row along the south fence. A row of chicken wire laid flat over the row keeps squirrels off until the seedlings push through.

May 28: emergence. Eight days, right on schedule for 62°F soil. Wire comes off.

June 10 to June 30: establishment. Deep watering twice a week. Plants hit 3 feet by June 25, and that's when the stakes go in, before the roots fill the space.

August 3: first bloom. That's roughly 75 days from sowing, which is typical for a giant type. Plants are 11 feet. Petals stay open and looking good for about 2 weeks.

August 18: petals dropping. Head starts to tilt forward. This is the clock that matters now: 30 to 45 days from petal drop to seed harvest, so the window is roughly September 17 to October 2.

August 25: cover the heads. Back of the head is just starting to pale from green toward yellow. Mesh bags go on now, before the birds notice.

September 12: check. Back of head is yellow-brown, bracts are drying, seeds are plump and striped but the very center seeds are still a little soft. Verdict: wait.

September 20: harvest. Back of head is fully brown, bracts are crisp, and center seeds are firm. That's 122 days from sowing, a bit past the 110-day packet number, which is normal in a cool summer. Cut with 12 inches of stem.

September 20 to October 8: curing. Heads hang in the garage, spaced a foot apart. After 18 days the backs are hard and seeds rub off easily.

October 8: threshing and drying. Seeds rubbed off, winnowed in front of a box fan, spread on baking sheets for 3 more days, then jarred and labeled.

The margin check. October 15 frost minus the September 20 harvest gives 25 days to spare. Comfortable. Had this gardener sown on June 20 instead, harvest would have landed around October 20, after frost and well into fall rain, and the seeds would likely have molded on the head. That's exactly why the last-sowing math above matters for giant varieties.

FAQ

When do you plant sunflowers for the best blooms?

Direct sow after your last frost once soil reaches 55 to 60°F, then sow again every 2 to 3 weeks through early July. A single sowing gives you one burst of bloom. Successive sowings keep flowers coming from midsummer to frost.

Can I still plant sunflowers in late August?

In zones 9 and 10, yes: sow a 55 to 65 day dwarf or pollenless variety now for fall bloom. In zones 8 and below you're out of time for flowers, since seedlings need roughly 60 frost-free days minimum. Save the seed for next spring and spend late August harvesting instead.

How long does it take a sunflower to grow?

Most garden varieties bloom 55 to 75 days after sowing. Giant seed types take 90 to 110 days to mature seed, with first bloom around day 75. Seed heads are then ready about 30 to 45 days after the petals drop.

When to harvest sunflower seeds?

When the back of the head has turned from green to yellowish brown, the bracts are dry and brown, the petals have dropped, and the seeds are plump with clear striping. Cut with 12 inches of stem and finish the drying indoors rather than waiting for the head to fully dry outside.

Do sunflowers transplant well?

Not really. Sunflowers grow a taproot that gets damaged during transplanting, which stunts the plant and makes it more likely to topple later. If you must start indoors, use peat pots or soil blocks that go in the ground intact and set them out before the seedling passes 4 inches.

How deep and how far apart should I plant sunflower seeds?

One inch deep in most soil, or 1.5 inches in sandy soil. Space dwarfs 6 to 12 inches apart, standard varieties 12 to 18 inches, and giants 24 to 36 inches. Sow a few extra and thin to the strongest, because bird and rodent losses are normal.

Will sunflowers come back every year?

Common garden sunflowers (Helianthus annuus) are annuals and die after seeding, though dropped seed often self-sows the next spring. If you want a perennial, grow Helianthus maximiliani or a perennial sunflower species instead. Leaving a few heads to shatter naturally is the easiest way to get volunteers.

Why did my sunflower head not fill with seeds?

Poor pollination is the usual cause, especially with pollenless varieties grown alone or in gardens with few bees. Drought during seed fill, extreme heat at bloom, and birds working the head early all do it too. Growing pollen-bearing varieties alongside pollenless ones fixes most of it.

For the rest of the fall calendar, see what to plant in September and our planting calendar by zone. If you're planning a big fall bed alongside your sunflowers, when to plant pumpkins runs on similar long-season timing.

Updated August 2026

A note on numbers. This page gives you soil temperatures, sowing depths, spacings and days to bloom, not dollar figures. Seed and tools are priced by your supplier and your region, and we do not set any of it. Every number here is one you take off a thermometer or a calendar.