Our onion, leek, and shallot sets have just arrived, which means it’s soon time to plant them! These flavorful plants are a clear favorite for cooking around the world and are surprisingly easier to grow than most expect. But there’s a first time for everything we understand, so with the recent arrival of these bulb producers, we thought we’d write an easy guide to allium care! So, don’t sweat about day length, fertilization, fungal disease, and soil types; we’ve got you covered!

Onion Day Length Guide Explanation

Figuring out and finding an onion that fits your area’s “day length” standards is initially intimidating. “Day length, that is, the duration of daylight hours within 24 hours, is influenced by the Earth’s tilt. Thus, various amounts of light reach different parts of the Earth throughout the year. Likewise, latitude influences what onions you can and can’t grow. Places with higher latitude (the closer you get to the poles), your day length is extended in summer” (Jameson). This is flipped for places closer to the equator in winter. Certain onions won’t survive beyond the equator, while others won’t survive near the equator, as onions are extremely light-dependent. Now, to keep it simple, our store typically only carries onion varieties suitable for Colorado, which typically has Intermediate-day summer growing periods. You can likewise grow Long-day onions further North in our state. To find out what day-length range your state is, click the link here: Click me!

Planting Time

“Plant onion sets from April to early May (for zone 4) or four to six weeks before your last frost date. If your soil isn’t workable, onion sets can be stored for roughly three weeks in a cool, dark, and well-ventilated space” (Dixondale). DO NOT put them in water.

Perfect Soil

One perk about onions is that they grow well in a variety of soil types! “So long as your soil has excellent drainage, to avoid rot, you can grow onions in sandy or clay-dense soil” (CSU). Just make sure to amend said soils with organic material to generate better aeration (especially for clay soils). We recommend organic materials, such as Cotton bur compost or humus, to help. The onions wouldn’t shy away from nutrient-dense soil! “To amend soil for onion beds, you’ll apply 1 1/2” of green compost or humus over the desired beds, and work said material 8” deep into the soil” (CSU). Likewise, the spot onions will be planted in MUST HAVE good sun (exemplified in the day-length paragraph).

Fertilizing

Start strong with fertilizing your onions by testing your soil. You’ll want a nutrient-rich soil with a balanced NPK ratio with no excess. “Onions require a good dosage of nitrogen; however, too much leads to early maturing and produces poor bulbs. While liquid fertilizers are fine for onions, slow-release solid fertilizers are better, especially if they’re organic. You’ll side-dress onions to fertilize them by scratching in the slow-release fertilizer above ground six inches away from the onion plants. You will want to do this after roots establish and only once or twice per growing season” (UM).

Watering

“Onion roots are shallow and thus sensitive to changes in moisture. Make sure to water frequently, never allowing their soil to dry out. If you allow them to dry during the bulbing stage, bulbs will be small and leathery” (CSU). Our 2025-2026 winter has been dry, and our summer will be too (most likely), so make sure to water your onion sets from now until harvest. An inch of water a week should be perfect, but keep a close eye on them regardless. “Cease watering entirely by August, or when the tops of the plant lie over on the ground. The food made in the leaves will supply the onion with nutrients from this point on” (CSU).

Fungal Issues & Insects

Sadly, it’s not all champagne and caviar with growing onions. “One infamous insect that loves onions are thrips” (CSU). These tiny flying lice-like bugs feed by puncturing leaves and sucking out the sap. They especially love onions, so non-systemic insecticides are essential for keeping them away and exterminating them. Onions also suffer from various fungal issues and diseases, from purple blotch to mildew. Always stick to organic fungicides when protecting onions and never systemics, otherwise you won’t be able to enjoy your crops! If you’re interested in learning more about onion diseases and fungal issues, check out a few articles featured below:

Disease & Fungus: Click me!

Insect Control: Click me!

Soil-Borne Diseases: Click me!

Weeds

Onions are susceptible to weed choking. “Get onion beds free from weeds with organic preemergents (such as corn meal), raking them into the top inch of the soil” (Dixondale). Preemergents will halt weed seed germination for six weeks, but established weeds will remain. Do not use any other herbicide and weed by hand with a hoe, dandelion popper, etc.

That’s just the basics when it comes to growing onions, shallots, and leeks from sets. For further questions, we highly recommend you check out the sources below or the others featured within this article. Feel free to visit the store for help from our staff or email Colorado State Extension Q&A for questions. Don’t let the onions frighten you, happy gardening!

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Jameson, Molly. “Understanding Short-Day vs. Long-Day Onions | Panhandle Agriculture.” Ufl.edu, 2024, https://nwdistrict.ifas.ufl.edu/phag/2024/07/26/understanding-short-day-vs-long-day-onions/

“1808 – Growing Onions from Seed – PlantTalk Colorado.” Colostate.edu, 2025, https://planttalk.colostate.edu/topics/vegetables/1808-growing-onions-seed/

“Growing Onions in Home Gardens.” Extension.umn.edu,https://extension.umn.edu/vegetables/growing-onions

“Onion Planting Guide – Dixondale Farms.” Dixondale Farms, 16 Apr. 2020, https://dixondalefarms.com/onion-planting-guide/?srsltid=AfmBOopg3208TqFNnzA7QA7bRfa1G1ElBS05UPDSmU2tczB506uMrrrB

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