Growing onions from seed opens up a wide variety of shapes, flavors, sizes, and colors. Here are some tips on how to choose varieties that will grow well in your area and how to start onions from seeds indoors under lights.
Onions are a staple in the kitchen and essential for adding flavor to numerous dishes all year long. Every year, one of my garden goals is to grow enough onions for our meals, to make sauces and salsas, and to store for the winter.
Onions can be planted from transplants, sets, or started from seeds. Onion transplants are sold in bunches and can be purchased online or at your local garden center. Onion sets are young bulbs that were grown the previous year. You can easily find them in the spring at garden centers and nurseries near you.
I remember planting onion sets as a child when my family put the garden in every spring. I bought scoops of onion sets and pushed them into the ground in the spring when I started my own garden, just like my mom did.
Overall, I didn’t have very good success with growing onions from sets. Some were duds that failed to sprout, and some ended up bolting before forming large bulbs. The onions that did grow from sets were rather small and lasted only a short time in storage. I also didn’t like that there weren’t many onion sets to choose from—mostly yellow, white, and red.
You can read our full growing guide, How to Grow Onions, to learn more about planting and growing onions. This article will show you how to grow onions from seeds indoors under lights.
Onions are a beloved vegetable in cuisines across the world. Their versatility, flavor, and health benefits make them a staple in most kitchens. While onions can be grown from sets, many gardeners prefer starting with onion seedlings which allows for a wider variety of cultivars. However, properly fertilizing the seedlings is key to growing hearty bulbs. This comprehensive guide will explore when and how to fertilize onion seedlings for optimal growth.
Why Fertilizing Onion Seedlings Matters
Onion seedlings require adequate nutrition from the time they germinate until harvest. Since seedlings have an underdeveloped root system, fertilizing them is crucial to strengthen the roots and spur foliage growth. The nutrients absorbed during the seedling stage carry the plant through its entire lifecycle.
Fertilizing also helps the seedling rapidly develop its photosynthetic capacity after transplanting. This powers the bulb enlargement process that follows. Insufficient fertilizer leads to stunted growth and reduced yields.
Ideal Time to Begin Fertilizing
Most experts recommend starting fertilizer application 3 weeks after transplanting the seedlings into the garden. At this stage, the roots are established enough to absorb nutrients efficiently. Avoid fertilizing earlier as the roots are still fragile.
Staggering your planting allows fertilizing at different times. However, ensure seedlings get their first feeding no later than 3 weeks after transplant regardless of planting date This could be as early as late winter in hot climates
Recommended Fertilizers
Balanced Blend (10-10-10): A nitrogen-phosphorus-potassium (NPK) balanced fertilizer is great for the initial seedling feeding. It supports overall root and leaf growth.
Nitrogen (21-0-0): Nitrogen is key for lush, green foliage. Begin applying a nitrogen fertilizer 3-4 weeks after the first feeding. Alternate nitrogen and balanced feed every 2-3 weeks.
Organic Options: Compost, compost tea, blood meal, bone meal, and pelleted chicken manure make excellent organic substitutes. However, their nutrient release is gradual compared to synthetic fertilizers.
Application Method and Quantity
How to Apply: Water the seedlings well before fertilizing. For concentrated synthetic fertilizers, dilute as per label instructions before applying. Side-dress granular fertilizer in furrows 2-3 inches from the seedlings and water in well.
How Much: Follow label directions for synthetic fertilizers. For organic options, a general rule is 1-2 cups per 10 ft row.
Frequency Fertilize every 2-3 weeks Increase frequency for sandy soils which drain faster
Foliar Sprays Liquid fertilizer sprays directly on leaves are great for quick nutrient absorption Spray monthly.
Testing Soil: A soil test indicates your garden’s nutrient levels and allows customizing fertilizer needs.
When to Stop Fertilizing
Cease all fertilizer application about 4 weeks before the anticipated harvest date. Feeding beyond this can impair the onion’s ability to cure and cause shorter shelf life.
The best indication that the seeds have finished bulbing is when foliage starts collapsing and drying up. The plant now begins redirecting energy from leaves to the maturing bulb.
Getting it Right
Pay close attention to the time of transplant, fertilizer ratios, organic or synthetic choice, application method and quantities when feeding onion seedlings. Aim for consistent moisture and sunlight as well. Adhering to these best practices will ensure your seedlings transform into bumper crops of picture-perfect onions.
Step 4: Add Light
Once the seeds sprout, remove the humidity dome and place the containers under lights in a cool location. Onions prefer cooler temperatures, so there is no need for the heat mat after the seeds germinate.
Position the lights so they are about 2 inches above the seedlings, and adjust as the plants grow. Keep the lights on for 16 hours, and off for 8 hours. Don’t worry, artificial lights will not trigger the plants to prematurely bulb.
Step 7: Harden Off
Begin hardening off onion transplants about 4-weeks before your last expected frost date. Hardening off means getting plants used to the outside so they can handle sunlight, cool nights, and less frequent watering. On the first day, harden off the onion seedlings in a protected area for a few hours. Each day, add more time until the plants are outside all night. Learn How to Harden Off Transplants with this full tutorial.
THE PERFECT TIME TO FERTILIZE ONIONS!
FAQ
Do you need to fertilize onion seedlings?
What is the fertilizer schedule for onions?
How do you take care of onion seedlings?
How do you grow onion seedlings?
Early Growth (Seedling Stage): During this initial phase, onion seedlings require a robust foundation for leaf development, which is crucial for subsequent bulb formation. Applying a nitrogen-rich fertilizer (such as a 10-5-5 N-P-K ratio) at about 0.5 pounds per 100 square feet encourages vigorous leaf growth.
When should you fertilize onion plants?
The ideal time to fertilize onion plants depends on the specific stage of growth. Here are some general guidelines: Pre-planting: Before planting onion sets or transplants, it is beneficial to incorporate organic matter, such as compost or well-rotted manure, into the soil.
Do onions need a slow release fertilizer?
Onions benefit from a slow-release fertilizer applied at planting time or a few weeks after planting. Avoid applying too much fertilizer, as it can lead to excessive leaf growth and reduced bulb development. Always follow the instructions on the fertilizer package for proper application rates.
Do onions need foliar fertilizer?
Foliar Feeding: Sometimes, plants need a quick boost of nutrients, especially if they show signs of deficiencies during critical growth phases. Applying a diluted liquid fertilizer directly to the onion leaves can be effective.
Are tomato fertilizers good for onions?
Tomato fertilizers are more balanced in their NPK ratio, for example the Expert Gardener Tomato & Vegetable Garden Plant Food Fertilizer from Walmart has an NPK ratio of 12-10-5. They can be beneficial to onions as both nitrogen and phosphorus are important nutrients for the plants.
What is the best fertilizer for onions?
Nitrogen promotes leafy growth, which is not ideal for onions as it can result in larger leaves and smaller bulbs. On the other hand, phosphorus and potassium promote root development and bulb formation. A good fertilizer option for onions is a balanced or slightly higher phosphorus and potassium fertilizer, such as a 5-10-10 or 10-20-20 blend.