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Grow Guide: Sunflowers

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.

Direct Sowing Seeds vs. Starting Seeds Indoors

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 to Start Sunflower 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.

Presoaking Seeds Before Planting

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.

Direct Sowing Seeds Outdoors

Prepping the Soil

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.

Seed Spacing

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.

  • Space giant sunflowers 2.5 to 3 feet apart. 
  • Space regular-sized sunflowers 1.5 to 2 feet apart. 
  • Space miniature sunflowers 6 to 12 inches apart. 

Regardless of the sunflower type, plant seeds about one inch deep and cover them with soil. 

Starting Sunflower Seeds Indoors

Supplies Needed

  • Seeds from the Sunflower Variety Pack
  • Growing media: Coconut coir and commercial potting soils are the two most common growing substrates used in containers, and both are available online or at local retailers. A quality growing medium in your containers is essential; it holds on to moisture and nutrients, provides air space around the roots, and anchors the plant’s roots to keep it upright.  
  • Containers: Seed-starting trays are the easiest to use, especially with domes or covers. The trays are wide and shallow, allowing you to plant many seeds in one tray. You can also use individual pots to start your seeds or recycle plastic containers from your kitchen (clean yogurt cups, sour cream containers, etc.). Just ensure they are cleaned and sterilized.
  • Plant tags: Use plastic or wooden plant tags to label what seeds you have planted and where. Plastic plant tags are more durable, and the words don’t fade as quickly; wooden tags are biodegradable and more environmentally friendly.
  • Supplemental grow light (highly recommended): LED, fluorescent, and compact fluorescent grow lights are good choices for hobby growers. Grow lights emit different wavelengths or “colors” of light crucial for plant growth. 
  • Seed starting mat (highly recommended): A seed-starting mat is used to speed up germination. It is similar to a household heating pad in size and shape and goes underneath a seed-starting tray to warm the growing medium.

Reusing Potting 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.

Prepping Growing Medium

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.

Step-by-Step Planting Instructions

  1. Fill containers almost to the top with pre-moistened growing medium.
  2. Plant seeds at a depth of about 1 inch, then cover with growing medium. 
  3. Mist the potting soil with room-temperature water.
  4. Cover the container by placing a lid or a plastic storage bag over it to increase the humidity.
  5. Set containers on top of the seed-starting mat or somewhere slightly warmer than the air temperature in your home, such as on top of the refrigerator.
  6. Periodically mist potting soil to moisten the medium without disturbing the seeds.
  7. If heavy condensation collects on the inside of the lid or bag, remove it for a bit or prop it open to allow air movement.
  8. Once seeds germinate and sprout, take the cover off to prevent damping off and move the container(s) to a spot with plenty of light.

Hardening Sunflower Seedlings Before Transplanting Outdoors

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.

Transplanting Seedlings Outdoors

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.

  • Space giant sunflowers 2.5 to 3 feet apart. 
  • Space regular-sized sunflowers 1.5 to 2 feet apart. 
  • Space miniature sunflowers 6 to 12 inches apart. 

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. 

Caring For Your Sunflower Seedlings


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. 

  • South-facing windows get the most sun; they typically have light streaming through them for most if not all of the day. 
  • West-facing windows stream direct sunlight for a good portion of the day and are often shaded from the intense sun in the late afternoon. 

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.

Temperature Requirements

Sunflowers thrive in warm conditions and need plenty of hot days to grow tall with large, beautiful heads. 

  • When planting outside, keep in mind your local spring conditions. Wait until the threat of frost has passed. If your region is prone to cool, rainy spring weather, wait until the rain clears up before planting. 
  • If you start seed indoors, keep the ambient air temperature between 70 and 78°F.

Watering Needs

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. 

  • For outdoor seedlings, water them lightly every 2-3 days, soaking the soil 3 to 4 inches out from around the plants well. Avoid spraying the plants directly with the hose stream and dislodging them from the soil.
  • For indoor seedlings, place the container(s) in a shallow pan of water to let them wick up the water they need. Bottom watering helps prevent overwatering and won’t dislodge the seeds or seedlings like overhead watering may.

Indoor Watering Concerns

Tap water is usually acceptable for indoor plants, but there are a few things to keep in mind:

  • If your home has hard water, you will see a white buildup on the soil surface over time. This buildup comprises calcium and magnesium salts from the limestone in the water. Periodically flush the soil with filtered or tap water to remove.
  • If you have softened water, collect water from your pipes before it goes through the softener or water your plants distilled or bottled water.
  • If the local municipality treats your water, it may have a reasonably high chlorine content, which can be problematic in sensitive plants. Before watering your plants, fill a watering container and let it sit for 24 hours to allow the chlorine to dissipate.


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.  

Troubleshooting Common Problems with Sunflower Seedlings

Damping Off

Damping off is a problem caused by fungus or mold that thrives in cool, damp conditions, resulting in seedling death. 

  • It is hard to avoid damping off in the garden because you can’t control the weather. If the weather changes and temperatures drop below average, immediately scale back on watering. You can also cover seedlings with individual cloches as an extra measure of protection.
  • Indoors, the most effective way to avoid damping off is to increase air circulation by setting up a small fan nearby. Keep the potting soil moist while avoiding overwatering, and keep containers in a warm location. 

Leggy Plants

When seedlings grow tall and spindly, it’s a sign they aren’t getting enough light.

  • There isn’t much you can do for plants grown outdoors to change their light exposure other than ensuring nothing is casting shade on them during the day.
  • If you start seeds inside, try moving the plants to a spot that gets more sun or supplement the lighting with a grow light. If you are using grow lights, they may be positioned too far from the plants. Place shims (books work well) under the trays so the plants are closer to the light source. The lights should be four to six inches above the tops of the plants. Some seedlings grow faster than others, so stacking smaller shims works well, making it easy to maintain the proper spacing.

Discolored Leaves

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.


  • Garden-grown sunflowers have far fewer insect pest problems than many other garden plants. But keep an eye out, though, especially as heads develop, for beetles, weevils, and earwigs. If you are growing tomatoes nearby, watch for cutworms, which will travel from tomatoes to sunflower plants. Early treatment of insects is vital, so regularly scout for pests and begin treatment immediately if any are spotted. 
  • Indoor plants have fewer pest problems than outdoor plants, but fungus gnats are problematic. You may have problems with aphids and spider mites, but they typically prefer larger plants with bigger leaves. Most pest problems can be discouraged with increased air circulation and correct watering; insecticidal soap or neem oil can also be used as a treatment.