Knowledge from Olle Garden Bed: Four Reasons For Empty Sunflower Seeds
Sunflowers are the most popular in the garden because of their beauty and delicious, nutritious seed harvest. Therefore, it may be disappointing to find that some (or many) seeds are actually empty. The following content also has some reference value for raised garden beds.
Poor seed filling (=seed development and growth in the shell) is not only a problem for family gardeners, but also a struggle for sunflower farmers who want high yield seeds or oil. The research on the embryonic development of sunflower seeds is still in progress, but there are some possible culprits responsible for empty sunflower seeds.
- High temperature or drought stress after pollination
It is well known that too much high temperature or drought will reduce the yield of vegetables, and sunflowers are no exception. Heat waves or droughts can put pressure on plants and reduce seed filling.  The process of seed development is very complex, involving the production and movement of photosynthetic products and the internal process of the seed itself.
If you have a period of very hot and dry days when you pollinate sunflowers, this may be a factor that causes you to have empty seed shells.
Although direct seeded sunflowers have deep taproots, if you expect a heat wave, make sure you water your sunflowers adequately. You can also use sunshade cloth to provide temporary protection, but this may be impractical for tall sunflowers.
- Worm damage
If you notice a small hole in an empty sunflower seed, it is probably a worm problem. Sunflowers have considerable resistance to pests, except for a few insects. The most problematic larvae lay eggs on flower heads, and their larvae actually eat the developing seeds inside.
The culprits are the red sunflower weevil (Smicronyxfulvus) and the gray sunflower weevil (Smicronyxordidus). The first two actually eat the growing seeds directly, then chew the shells and fall to the ground, where they will find a place to live through the winter and become adults in the next summer.
Unless you have serious worm damage, it is usually not worth spraying sunflowers with organic pesticides (if you do, it is better to apply them when the flowers are just opening), because these pesticides can kill beneficial pollinators and parasitic bees, both of which help to increase sunflower seed yield.
- Phosphorus and/or nitrogen deficiency
Lack of nutrition will lead to more seed filling problems in sunflower seeds. Phosphorus is one of the key macronutrients of all plants, together with nitrogen and potassium, but it is particularly important for fruit and seed development. However, nitrogen also plays a key role, particularly in relation to seed filling. 
Nitrogen is more important for the growth of leaves and stems. However, seed filling depends on photosynthesis. Nitrogen can ensure the healthy leaves and metabolism of sunflowers. The results showed that increasing nitrogen had a positive effect on sunflower yield, especially during seed filling period.
Feed sunflowers with balanced or high nitrogen fertilizer as early as possible, while focusing on leaf growth. Use quality slow release fertilizer applied monthly (or as instructed) or gently feed fast acting liquid fertilizer every 7-14 days. When the sunflower plant reaches maturity, please balance the fertilizer or use more phosphorus fertilizer
- Lack of solar energy (weak light or fallen leaves)
Therefore, nitrogen is important for providing your sunflower with healthy enough leaves to drive photosynthesis, which is important for the development of seed/kernel. The next logical step is to assume that sunflowers with fewer leaves or insufficient sunlight will have more empty seeds.
Facts have proved that some studies have examined the effects of defoliation (removal of leaves) on sunflowers. It is found that not only the yield is reduced, but also the seed filling is reduced, leading to partial seed development or even hollowing. 
You may not have to worry about sunflower leaves, but consider where sunflowers are grown and how much sunlight they get. For sunflowers, the more sunlight, the better. If you plant sunflowers in rows or several trees together, some of them may be covered, resulting in lower yield and more empty seeds. For large sunflower varieties, it is best to keep them about 3 feet (1 meter) apart, but you can plant them closer and still get a good harvest. If you plant a line, try to plant it from east to west, so that they can get enough sunshine from the south. Avoid planting sunflowers near fences or walls facing north.