If soil temperatures are just right, sunflower seedlings will sprout up in 10 to 14 days. Growing sunflower seeds requires space. To start sunflowers indoors, plant three seeds per each 3- to 4-inch peat pot.
The soil was too wet or too dry:
Too dry, and your seeds won't germinate. Too wet, and they're liable to rot. Ensure that seeds are kept evenly moist by thoroughly wetting and draining the compost before you begin.
When to Plant Sunflowers. Plant seeds after the danger of spring frost has passed and the soil temperature is at least 60 degrees. This will be between March and May, depending on where you live.
Sunflowers can be sown straight in to the ground where they are going to flower, so make sure the space you are going to sow is weed free, by using a trowel to remove the weeds. Rake the soil to a fine tilth (a fine crumbly texture) and make some drills 12mm deep. Leave a 10cm space between each seed.
Small sunflowers may require only 6 inches between each plant, while large varieties might need up to 3 feet. Closer spacings are possible for garden aesthetics, but crowded plants will produce smaller flowers.
Some sunflowers are annual and survive only one season, and others are perennials that will return every year. All perennial and annual sunflowers produce brilliant flowers that are a highlight of any late-summer garden.
The narrow end of the seed is where roots will emerge, so putting this end down saves the plant from having to right itself in the ground. Place the sunflowers 1 to 2 inches deep, point-down, every 6 inches or so, and then cover them up.
Sunflowers and Light Requirements
Sunflower seeds do not require light for germination. However, when grown indoors in pots, the plastic wrap is removed at the first sign of germination and the uncovered pots are placed in bright, indirect light.
Place one seed in the divot and cover it with soil. If you have extra seeds, you can put 2-3 in one hole and then trim off the two weaker seedlings. Water the soil gently to help settle it. Sunflower seeds of most varieties should be planted about 18-24 inches apart, as they will grow to be quite large.
So how late can you plant sunflower seeds? With varieties that only require a 50-60 day growing season, you should be able to plant sunflowers as late as July for fall flowers. If you live in a warm climate where the first frost comes well into late fall, you can plant sunflowers even later in the season.
The allelopathic characteristics of sunflowers are toxic to some plants, potatoes being one of them. This process inhibits germination and can stunt the growth of potatoes. They also compete for nutrients in the soil. Potatoes should be planted well away from sunflowers to avoid them affecting one another.
You can still plant flowers, too!
Planting later crops in June also applies to flowers! Plant more marigolds, sunflowers, cosmos, zinnias, four o'clocks, nasturtiums, etc. – all of the annuals that usually burn out by late summer.
Soak Sunflower seeds with warm water (40-50 C) for 25 minutes with the aim of breaking the seed dormancy period (awakening the seed while accelerating germination).
The good news is that they are easily removed by soaking (and then a good rinse). And, although invisible to the naked eye, when soaked for 12 – 24 hours the sunflower seed starts the process of germination. This germination activates nutrients within the seed and increases its digestibility.
Now is not the time for sentimentality. Crowded plants not only discourage growth, they encourage pests and disease. Crowded seedlings shade each other from the sun. As they get larger, it only gets worse.
The fastest germinating seeds include everything in the cabbage family – bok choi, broccoli, kale, cauliflower etc, and lettuce. The slowest seeds to germinate are pepper, eggplant, fennel, celery, which may take 5+ days.
Overnight is usually good. Many sources recommend 8-12 hours and no more than 24 hours. Again, too much soaking and the seeds will start to decompose. If you use very hot water, the soaking time will decrease.
Sunflower seedlings should be watered daily so the soil is moist but not soaked. As the sunflowers become established, they can be watered once a week. This watering is infrequent but it should be a deep watering that uses several gallons of water.
The recommended planting depth for most sunflower seeds is about 1/2 inch deep, and potentially down to 1 inch deep for varieties with larger seeds or in soil that dries out quickly. However, it's important to check the specific instructions on the seed packet, as some varieties may require differing depths.
In the morning, all the sunflowers will face east, the direction of the rising sun. As the sun moves, so will the sunflower heads to follow it. But as summer turns to autumn and the sunflowers get big and ripe and heavy with seeds, they will stop moving. Until they are harvested, they will face only east.
Sunflowers are fast reproducers, and one plant can create dozens of others. Plant seeds about an 1 inch below the ground. Germination occurs quickly, usually between 5 and 10 days, but spring crops produce more plants than summer crops. Spring crops are planted in April and May.
Once your sunflowers have died back completely and the backs of the blooms are brown, it's time to harvest. You'll also notice the seeds are plump and somewhat loose. Cut the stalk with sharp scissors or pruners, about one foot down from the flower head, and place in a container that can catch any loose seeds.