What Is the Height of Sunflowers After 3 Weeks?

Growing Pains

• Sunflowers sown from seed typically reach 4 to 12 inches in height after three weeks. Their thick roots tunnel through the soil in search of natural resources, and by 7 to 11 weeks, flowers blossom, depending on the variety. However, heights fluctuate considerably when environmental conditions are poor. Cold weather and drought conditions often stunt sunflowers and force them to remain without blossoms until resources such as nutrients and oxygen in the soil, are ample.

Fast and Steady Growth

• If you germinate your sunflower seeds in flats exposed to 70 to 75 degree Fahrenheit temperatures, your 3-week-old sunflower's height may be closer to 12 inches because of ample warmth. Seeds germinate quickly in these temperatures, emerging after 2 days in some cultivars. Even seedlings older than 3 weeks tend to grow taller in a shorter amount of time when the temperature is kept between 70 and 75 F. Growth typically slows when temperatures rise higher than 75 degrees F. Exposing sunflowers to 65 degree F. nights may slow their growth slightly, but this cooler temperature also encourages a steady increase in height, unlike the quick but unsteady growth of sunflowers exposed continually to warm conditions.

Time to Grow

• With foliage emerging first, established seedlings need a full sunlight location and ample spacing for successful root growth and air circulation around leaves. These prime environmental conditions allow your sunflowers to grow steadily to their mature height. If you grow sunflowers indoors under artificial lighting, maintain 13-hour days because sunflowers exposed to shorter days resort to rapid flowering but grow poorly. The sunflowers detecting waning light, which in sunflower-speak means the growing season is ending, produce flowers to keep the species reproductively successful.

Water Deep

• From germination to seedling establishment, sunflowers require consistently moist soil conditions for normal growth. Well-drained soil coupled with infrequent irrigation sessions encourages deep root growth. Roots that rapidly spread provide abundant nutrients, moisture and oxygen to the stem, foliage and flowers. Use a drip irrigation system that seeps water into the soil directly instead of overhead or sprinkler components. You avoid waterlogging the soil with accurate drip irrigation and save water at the same time. Foliage remains dry which impedes pathogen and pest issues as the sunflower grows tall.