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 slowly over several passes so that you don’t get rid of too many seedlings. First, cut down to double the desired spacing. After two days of growth, thin again until the plants are evenly spaced. 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.
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. If you plant them in an area with a lot of deer, you might want to think about fencing or other ways to keep the deer away.
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.
Sunflower yield and quality can go down because of some diseases, like Alternaria and Phoma leaf spot, Rhizopus head rot, rust, and white mold. 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. They are sometimes used as a “trap crop” to keep bugs and other pests away from other plants because they are tasty to animals and bugs. 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 |
Comments |
|
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. You should because if you don’t, none of your plants will have enough space to grow to their fullest size. 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.