How to Thin Sunflower Seedlings for a Healthy Crop

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Robby

Are you looking to add something aesthetically appealing to your garden? Do you want something that attracts pollinators and is a good food source for animals and humans? If you answered yes to any of these, you should consider adding sunflowers to your garden. Sunflowers are a beautiful and versatile addition to any garden. They are one of the few crops that will meet both your ornamental and oilseed production needs.

Sunflowers can be grown from seeds and come in a variety of color patterns and sizes. Typically, sunflowers stand about 6 feet tall, but some varieties can grow anywhere from 2 to 15 feet tall. By utilizing succession plantings, you can harvest them throughout summer and fall.

Even though sunflowers are one of the only crops native to the United States, they didn’t become widely produced until the 1970s due to the demand of sunflower oil in Europe. The versatility of this crop allows it to thrive in a wide variety of soils and climates.

During the growing stages, young sunflower heads track the sun to absorb as much light as possible in a process called heliotropism. Once flowers become mature, the stem stiffens, and the sunflower heads will generally all face east.

Two types of sunflowers are typically grown – oilseed varieties and confection sunflowers. Oilseed sunflowers are grown for sunflower oil and bird seed. Sunflower oil is considered a healthy oil since it is low in saturated fats. Confection varieties are grown for human consumption, often in the form of roasted and baked seeds.

Sunflowers have extensive root systems that help them easily utilize soil nutrients. Sunflowers prefer well-drained soils with a pH between 6.0 to 7.0, like clay loam and silty clay soils. Test your soil to determine pH level and fertility needs before planting sunflowers.

Once the second set of leaves have appeared, a slow-release all-purpose fertilizer can be applied directly to the plants to speed up growth and produce larger flowers. Sunflowers also can benefit from applying half the recommended nitrogen during planting and the second half later in the growing season, which is known as a split application.

Sunflowers are annual plants that perform best in sunny locations. They can be easily started from seed by directly placing seeds into the ground. Sunflowers can also be started indoors as transplants, which allows for them to bloom earlier than direct seeding.

Plant sunflower seeds 1 to 2 inches deep after the last threat of frost. Space seeds 6 inches apart for smaller varieties and 12 inches apart for taller varieties. Once seedlings emerge, be sure to thin out as needed. Rows need to be spaced 2 to 3 feet apart.

As a gardener growing sunflowers from seed, you know that sunflower seeds are easy to start indoors or direct sow outdoors. Though sunflower seeds readily germinate, this can result in overcrowded seedlings that must be thinned for the best growth. Thinning sunflowers is an important garden task that reduces competition between plants for sunlight, nutrients and water. Thinned sunflower plants grow stronger, produce more flowers, and have a higher seed yield.

When to Thin Sunflower Seedlings

Sunflowers are rapid growers that quickly develop a strong taproot. Thinning should occur early, when seedlings are 2-4 inches tall and have 2-4 true leaves. Waiting longer allows seedlings to become crowded, resulting in spindly, weak plants. Thinning is easier on small seedlings and causes less root disturbance than waiting until plants are larger.

The proper time to thin is based on seedling size and growth stage, not the number of days after planting. However, thinning typically occurs 1-3 weeks after germination when started indoors or directly sown outdoors after danger of frost has passed.

How to Thin Sunflower Seedlings

Thinning reduces the number of plants to the recommended spacing for the sunflower variety. Check seed packet plant tag or seed catalog for ideal spacing, as sunflowers range in mature size from 2 feet to over 12 feet tall. Dwarf sunflowers can be spaced as close as 4-8 inches while giant sunflower spacing may be 1-3 feet.

Follow these tips for thinning sunflower seedlings

  • Water seedlings well the day before to soften the soil and make pulling easier.

  • Select the strongest, healthiest plants to remain in the row or planting bed.

  • Grasp weaker, overcrowded seedlings near the soil line and pull to remove.

  • After thinning, spacing between remaining plants should be uniform.

  • Water immediately to settle roots of remaining seedlings.

  • Consider saving pulled seedlings to transplant elsewhere, if roots are not damaged.

  • Discard pulled plants or add to the compost pile.

How Much to Thin Sunflower Seedlings

Thin seedlings to the number of plants appropriate for mature plant size and garden space. Final spacing for dwarf sunflowers may be 6-12 inches, while giant sunflowers require 1-3 feet between plants. Specific thinning recommendations based on sunflower type:

  • Dwarf sunflowers: Thin to 6-12 inches between plants.

  • Medium sunflowers: Thin to 12-18 inches between plants.

  • Giant sunflowers: Thin to 18-36 inches between plants.

  • Multiple heads: Thin to 12-24 inches between plants.

  • Rows: Thin each row to spacing for variety, with 1-3 feet between rows.

Do not be afraid to thin vigorously. Removing more than half of the seedlings results in healthy, robust plants that produce an abundant harvest.

Avoiding Excess Thinning

Thin gradually in multiple passes to prevent removing too many seedlings. First, cut down to double the desired spacing. After a couple days of growth, thin again to the final spacing. This prevents eliminating more seedlings than necessary.

To avoid over-thinning, mark the desired spacing with stakes or plant labels when seedlings are small. Thin only the seedlings not near a marker. Move markers to the remaining seedlings to mark the next thinning.

Benefits of Thinning Sunflower Seedlings

While thinning requires cutting down healthy seedlings, the benefits of proper thinning result in:

  • Strong, stocky sunflower plants with larger flowers.

  • More heads per plant, boosting flower production.

  • Increased pollination, fertilization and seed yield.

  • Larger, plumper seeds.

  • Reduced pest and disease problems due to improved air circulation.

  • Allows proper watering and fertilization of appropriate number of plants.

By thinning crowded seedlings, you give sunflowers the space they need to grow into sturdy, high-yielding plants. Follow proper thinning practices and you will be rewarded with a bountiful harvest of bright, beautiful sunflower blooms and seeds.

how to thin sunflower seedlings

Starting Seeds Indoors and Transplanting

It is best to plant seeds indoors in a controlled environment in biodegradable containers. Biodegradable pots allow the entire pot to be planted in the ground without disturbing the established root system. Before planting outdoors, the seedlings will need to be “hardened” to acclimate to the outdoor environment. One week prior to planting, place transplants outdoors in a safe location for a few hours. Each day, gradually increase the amount of time transplants spend outdoors. After several days, transplants should be spending 24 hours outdoors and will be ready to be planted into the ground once the threat of frost has passed.

Sunflower varieties typically reach maturity in 85 to 95 days. Consider using succession plantings to have sunflowers blooming throughout the entire growing season. Succession planting can be accomplished by planting the same variety of sunflowers every two to three weeks, planting different varieties with different maturity dates at the same time, or planting multiple varieties with multiple maturity dates every two to four weeks.

Sunflowers can handle warm temperatures and some drought, but they do need watered regularly to help create a strong root system during the beginning stages of growth. Sunflowers also need watered when applying fertilizer during the growing season to allow the fertilizer to move to the roots for uptake. Once plants have a strong start, sunflowers can be watered occasionally if there is no rain for several days. Sunflowers only require an inch of water a week.

Sunflowers can only outcompete weeds once they start to mature, therefore weed control is extremely important in the early stages of growth to allow plants to obtain nutrition and moisture. You can control weeds by tilling, hoeing, hand-pulling or applying mulch around the plants. You can also use herbicides to control weeds, but be sure to follow directions on the container label.

Although sunflowers are easy to grow, they can fall victim to wildlife, insects and diseases. When selecting your location, be sure to choose a spot that is safe from potential deer damage. Deer enjoy young sunflower plants and can demolish them quickly. Fencing or other deer repellents will need to be considered if planting them in an area with a high deer population.

Birds are another pest when growing sunflowers because they like to eat the seeds. They can be deterred by using spinners, scarecrows or owl decoys.

A few diseases, such as Alternaria and Phoma leaf spot, Rhizopus head rot, rust and white mold, can cause both yield and quality reduction of sunflower. In addition, powdery mildew and downy mildew can also appear on sunflowers at a later growth stage. Infection and spread of most of these diseases is favored by warm, humid conditions and extended periods of leaf wetness. Early planted fields are generally more susceptible to severe disease losses and may need measures against diseases. Cultural practices, such as rotation, deep plowing of crop residue and proper spacing, can reduce disease severity without a need for fungicide spray.

Insects can also be an issue when growing sunflowers. Since they are appealing to wildlife and insects, they are sometimes used as a “trap crop” to keep insects and pests away from other plants. Keep in mind that sunflowers also attract beneficial insects, such as bees and butterflies. When applying insecticides be sure to use caution and follow the label.

Insect

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Leaf-footed bugs

Eats plant stems, leaves and flowers.

Stink Bugs

Piercing and sucking mouthparts that can cause severe damage to plants and crops

Caterpillars

Chewing mouthparts and can devour leaves and flower petals

Aphids

Use slender needle-like mouthparts to feed on the sap from the plants. They are very common and can be found on most plants in gardens.

When the back of the sunflower seed head starts to turn brown, that is a sign that it’s time to harvest your sunflowers Combines are used to harvest large fields of sunflowers. There are specifically designed combine heads, or the regular “all crop” platform head is sufficient, but you can expect to have seed loss with an unmodified platform head. Some producers will modify the headers and install “catch pans” to help catch some of the seeds.

Home gardeners can harvest their sunflowers in the early morning to use for floral arrangements. Handle flowers carefully, and they can last in water at room temperature for up to a week. To harvest the seeds, you can either cut the flower heads off and let them dry and then remove the seeds or let them dry on the stem.

Sunflower seeds supply healthy unsaturated fats as well as magnesium, copper, manganese and vitamin E. One tablespoon of sunflower seeds contains 51 calories, 2 grams of protein, 4.5 grams of fat and 2 grams of carbs.

Resources

Daniels, P. (2018, October 02). Seeds are healthy sources of fiber. Retrieved March 22, 2021, from https://www.canr.msu.edu/news/seeds_are_healthy_sources_of_fiber

Trott, R. (2020). Sunflowers. Retrieved March 22, 2021, from https://extension.umn.edu/flowers/sunflowers#branching-cultivars-2103960

Westerfield, R. (2017, November 20). Growing Sunflowers in the Home Garden. Retrieved March 22, 2021, from https://extension.uga.edu/publications/detail.html?number=C1121&title=Growing+Sunflowers+in+the+Home+Garden

Authors: Natasha Harris, former WVU Extension Agent, and Jesica Streets, former WVU Extension Agent

Last Reviewed: March 2022

How to Thin Sunflower Seedlings

FAQ

When should you thin sunflower seedlings?

If the soil is kept moist, seedlings will appear within 5-10 days. When the plants grow to 3 inches, thin them to the most vigorous 3 or 4.

How to thin out crowded seedlings?

The easiest way to thin seedlings is to use scissors. Just trim the excess seedlings off at the base of the plant, as close to the soil as you can get. The roots will die back and leave your remaining seedling undisturbed.

What happens if I don’t thin sunflowers?

I know it’s rough, but the answer is yes. Because here’s what happens if you don’t: None of your plants will have the room they need to grow to their fullest potential. Overcrowded plants often grow leggy (they basically grow too tall and thin in their search for more resources) and show signs of stress.

How to strengthen sunflower seedlings?

Annual sunflowers need plenty of water and will suffer if allowed to dry out. If you’re growing for height, feed them fortnightly with a nitrogen-rich fertiliser and then switch to a potash-rich tomato feed just before flowering.

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