Starting seeds may seem intimidating to a beginner, but it's a reasonably straightforward process. This guide discusses everything you need to know to start sunflower seeds. It walks you through all of the supplies you’ll need, teaches you how to prepare for planting, gives you step-by-step instructions for sowing, explains how to care for your new seedlings, and helps troubleshoot some of the most common problems.
Unlike many garden plants best started indoors, sunflowers do best when direct-sown right into your garden soil. As sunflowers grow, they develop a long taproot, and that taproot doesn’t like to be disturbed. So once they’re planted, they want to stay in one spot. That said, you can start seeds indoors successfully—you’ll just need to make a few slight changes compared with how you start other kinds of seeds.
When direct sowing seeds:
Plant sunflowers inlate spring after all threat of a late-season frost has passed. Germination occurs for most sunflower types when soil temperatures are between 70 and 85°F. If you can check soil temperature, sow seeds once temps reach 60 to 65°F.
When starting seeds indoors:
If you opt to start seeds indoors and then transplant them outside, start them about four weeks before the last spring frost date for your area.
Soaking seed before planting is a common way to improve germination. Seeds naturally have a hard outer layer to protect themselves from insects, diseases, and unfavorable germinating conditions. Soaking them in water softens the hard seed coat, exposing the embryo to moisture and kickstarting growth.
Sunflower seeds don’t need to be soaked before planting—after all, the seeds split easily—but soaking them for 12 to 24 hours will hasten germination and increase the number of seeds that sprout.
Sunflowers are tolerant of many soils and seem to grow almost anywhere a seed can land and germinate. When growing them in your garden, you can plant seeds with little prep work, but they will do much better if you work the soil before planting.
Cover the planting area with a couple of inches of finished compost or aged manure. Using a shovel, garden rake, or rototiller, work it into the ground, loosening the top 12-18 inches of soil. Crumbly soil without any hard soil clods allows the sunflower’s long taproot to grow easily and also improves water infiltration.
Spacing depends on the type of sunflowers you are growing and their estimated mature height. Taller plants need more space between the seeds, but you can plant smaller varieties closer together. You can plant seeds closer together and thin the seedlings to the desired spacing when they have two to three sets of leaves.
Regardless of the sunflower type, plant seeds about one inch deep and cover them with soil.
You can reuse potting soil from past gardening ventures, but it should be sterilized to remove pathogens or fungal spores, especially if you’ve had trouble with seedlings damping off in the past.
To sterilize your potting soil, thoroughly moisten it and place it in an oven-safe dish no more than three inches deep. Cover with aluminum foil and place in an oven preheated to 200°F. When the soil's internal temperature reaches 180°F, bake for thirty minutes without opening the oven door. Shut the oven off and keep the soil inside until it reaches room temperature.
It’s best to pre-moisten the medium before filling your containers. The potting soil should feel damp but not soggy. It should hold together in a lump without excess water dripping if you squeeze a handful.
Moisten the potting soil by putting some in a large basin or bucket and adding lukewarm water. Use a clean trowel, serving spoon, or your hands to mix it thoroughly. Add more water or potting soil until you reach the desired dampness.
About a week before you hope to move seedlings outside to the garden, start acclimating them to outdoor conditions. This process, called “hardening off," helps minimize transplant shock from severe temperature variation and light-exposure differences. Start by setting the planting trays or containers outside in a sunny spot protected from the wind for a few hours. Gradually increase the length of time the plants are outside every day, bringing them in at night, until it’s time to transplant.
When you transplant your sunflowers outside, disturb or manipulate the root system as little as possible. Dig holes at the following recommended spacing. The holes should be about twice as deep as the containers and 1.5 times as wide.
When the holes are ready, gently break apart the bottoms of the peat pots and place the containers so the tops are slightly below the soil surface. Backfill with the soil you removed, tamping down to eliminate air pockets.
When grown outdoors, sunflowers need a full-sun spot that gets a minimum of six to eight hours of bright sunlight daily.
Indoors, seedlings need a spot that gets twelve to sixteen hours of bright daylight. The best places are close to south- or west-facing windows.
There is a high likelihood the light inside your house isn’t adequate for good growth, especially in the winter when there is less daylight. To compensate for low light and prevent spindly, leggy plants, supplement the natural light using a grow light.
Sunflowers thrive in warm conditions and need plenty of hot days to grow tall with large, beautiful heads.
Once seedlings have sprouted, keep the soil or growing medium moist without overwatering.
About six to eight weeks after germination (after you transplant indoor seedlings outdoors), you can scale back on watering. Instead of keeping the soil slightly moist, allow the soil to dry out slightly and give plants about an inch of water once or twice a week.
Tap water is usually acceptable for indoor plants, but there are a few things to keep in mind:
How much you fertilize your sunflowers is a personal preference. Wild sunflowers often grow rampantly in poor soils, so it isn’t necessary to fertilize them when grown in fertile garden soil—especially if you worked compost into the soil before planting. They will, however, develop larger heads and grow taller when fertilized regularly.
There is no need to fertilize your seeds before they sprout. Inside every seed is an endosperm that stores food, usually starch and some nutrients, for the developing embryo. This “reserve” is enough to get the seedling started, especially since the plant’s needs are low due to its size.
After your seedlings develop the second (or third) set of true leaves, fertilize them once a month using a balanced, water-soluble fertilizer, liquid seaweed extract, compost tea, or a fish emulsion.
Damping off is a problem caused by fungus or mold that thrives in cool, damp conditions, resulting in seedling death.
When seedlings grow tall and spindly, it’s a sign they aren’t getting enough light.
Since the seed contains only a small amount of food, young plants discolor when these reserves deplete. A lack of phosphorus causes leaves with a reddish-purple hue. Yellow leaves can indicate your seedlings need nitrogen. To fix discolored leaves, apply a half dose of diluted plant food.