Aptly named for its sun-following blooms, the sunflower (Helianthus annuus) originated in North America and belongs to the daisy family. The big, yellow-gold flowers appear throughout the summer and, depending upon the cultivar, have big brown, green or yellow centers full of seeds. Sunflowers are warm-season annuals and grow in most climates. Sunflower seeds germinate readily when started at greenhouses before the beginning of the growing season. With good care, these seedlings with rise to healthy, mature plants.
Temperature and Light
As its name implies, the sunflower demands warmth and light to grow and flourish. Sunflower seeds germinate more readily at temperatures of 46 to 50 degrees Fahrenheit. A daytime temperature of 72 F and a nighttime temperature of 65 F will promote healthy seedling growth. Sunflowers subjected to temperatures above 78 F will develop long, leggy stems. Exposure to direct sunlight for at least 10 to 13 hours per day is best for these sun-loving plants. Provide artificial light when organic lighting fails to fulfill this.
Water
Although sunflowers are somewhat drought-tolerant, regular watering will guarantee appropriate development of leaves and stalks while they grow in the greenhouse. Consistently moist soil is vital during germination, since the seeds take in water to grow and swell. Seedlings do nicely when the top 1 inch of encompassing soil dries between watering. Soggy soil is very harmful to roots and shoots. Applying water straight to the soil in the morning is going to keep the sunflower leaf dry through the cooler nights, which will help minimize infection and rot.
Fertilizer
Potted sunflowers growing in greenhouses need regular fertilization to encourage their developing roots, leaves and stems. Without supplementary nutrients, the plants will quickly exhaust the supply from the soil. Nutrient deficiencies will quickly happen, manifesting as yellowing leaves, stunted growth and falling leaves. Bi-weekly uses of a 15-5-15 nitrogen, phosphorous, potassium fertilizer will offer the plants with the nutrients that they require. Start fertilizing sunflowers as soon as they develop two or three sets of true leaves. Apply the fertilizer in area of watering at a rate of 1 teaspoon of fertilizer a 1 gallon of water.
Maintenance
Sunflower seedlings that become extremely leggy will benefit from removal of the terminal bud. Clipping the very top of the main stem off just above the second group of leaves when the seedling grows four or five sets of true leaves will encourage lateral branching. Pinching sunflowers any after in their development may reduce or delay flowering. Transplant sunflower seedlings into 6-inch pots when they reach 3 to 4 inches tall. The larger pot will offer the roots with room to grow and develop into a sturdy support system. Staking potted sunflowers when they reach 6 inches tall will offer support for your stem.