Growing Onions 101
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!
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
2025 Garden To-Do’s Recap 1#
Here are the gardening references we shared over the past month. We’re big fans of Colorado State University extension, so you’ll see us referencing them all the time. However, we also have some written articles just for you under our blog! There will be more gardening references for the year, so stay tuned!
Relative Houseplant Care/Rescuing House Plants:
Listen…shhhhh…if you listen closely, you can hear that neglected plant in the corner crying for help. You did not mean to cause harm. Whether you left for your holiday travels in a rush and failed to give your plants some extra TLC before you left, or you do not know the plant’s needs, the stress is equally killing you. You are at the point of not knowing what to do to make your plants happy again. No worries! We got you on our blog!
Starting Seeds Indoors:
We know it seems early, but the sooner you start your seeds the better. For tomato and pepper plants it’s recommended you start them outside once soil temperatures reach 50 degrees at night. This however doesn’t work well for Colorado’s short growing season. To get the most produce out of your vegetables we recommend starting seeds indoors now and in February! All you need is a seed-starting soil mix and a grow kit!
Seed Starting Troubles:
Have you been waiting for your seeds to pop up and still nothing? No matter what you do, your seedlings perish. There are multiple factors as to why your seeds may not be sprouting. Seed age, crusting soil, and temperature fluctuation are just a few. In the article below by CSU Extension, scientists review why your seeds may not thrive as they should. Fear not, solutions are provided to fix issues and to get those seeds growing! Happy gardening!
Winter Watering:
If air temperature is above 40 degrees it’s time to winter water! Read more about winter watering rules below!
https://www.csu.org/blog/winter-watering-tips
Do’s & Dont’s of Transplanting:
Want to get bang for your buck when starting transplants? Learn the dos and don’ts of growing transplants by copying and pasting the link below! And if you need containers we wanted to remind you we have a free drop off/pick up bin behind the store in our parking lot-perfect for starting seeds or other transplants (once they’re clean that is)!
Garden Planning:
Now’s a great time to start garden planning for spring! Remember to consider space when mapping your beds and companion plants! The links below will lead to more information about early garden planning and companion plants!
General Info:
Companion Plants:
General Onion Care:
Intimidated by growing and planting onions? How about storage, before and after planting and growing these flavorful bulbs? And what the heck does daylight have to do with all of it?
Among the hundreds of questions involving onions you may have, we’ve contacted the experts for their advice. Learn the essentials for onion care from Dixondale Farms!
Growing Potatoes at Home:
Want to try growing potatoes this season in your home garden? We have a link for you! We recommend reading Colorado State University article on the subject!
https://extension.colostate.edu/topic-areas/yard-garden/potatoes-in-the-home-garden-7-617
Spring Bulb Care:
Intimidated by growing spring bulbs? Don’t be with Colorado State Extensions help! Learn how to keep beautiful spring bulbs, corms, and roots alive by reading the article below or in our story/highlights. The article provides everything from choosing a perfect planting spot to planting depth for certain favorites Happy gardening!
https://extension.colostate.edu/topic-areas/yard-garden/spring-planted-bulbs-corms-and-roots-7-411
Rick’s Tree Planting Guide:
We know many of you are familiar with our Tree Planting Guide, so here it is again! This guide is perfect for those interested in buying a tree or about to plant a new one. We also sell mulch, root stimulators, and mycorrhizae for tree planting! Happy gardening!
https://www.ricksgarden.com/wp-content/uploads/2021/04/Ricks-Tree-Shrub-Planting-Guide.pdf
