Onion Planting Guide

Onion Planting Guide

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Onion bulbs have been in cultivation since early historical records dating back to ancient Egypt. Onions can be used for a wide variety of dishes and they bring a large kick of flavor to everything. Onion greens can be used as scallions, the bulbs can be harvested when they are small and mild, or harvested when they are large and spicy. The can be grown in the garden, a raised bed, a container, or even for decoration in the flowerbeds.  When growing onions, selection of the correct varieties for growing in local regions is essential.  If you grow the wrong kind of onion in your area, you may not have much success.  Onions are categorized into three main groups: Short Day, Long Day, and Intermediate onions.  At Anderson’s, we only carry Long Day and Intermediate onions, which grow the best in our climate.


Onions prefer a loose, sandy soil that is rich in organic matter, well drained, and not too heavy.  They also need full sun exposure.  Before planting, incorporate 2-3 inches of well composted organic matter and 1-2 lbs of all-purpose fertilizer (we recommend “That’s All it Takes” complete fertilizer) per 100 square feet and work them into the soil to a depth of 4-6 inches.  Heavy, clay-based soils must be amended with compost and organic matter to encourage and allow good root development.  If you have heavy soil, we recommend 4-6 inches of organic matter and 50 lbs of Utelite or Zeolite per 200 square feet added to the soil each fall for multiple years to increase drainage and nutrient availability.  Over time, you can create a better growing environment for your garden plants to thrive in and produce.  Please see our information sheet “Preparing your Soil” for more detailed info on soil preparation before planting a garden.


Onions can be planted from seed or transplant. Plant transplants or sets in mid-March through April for early production (plants arrive in early March and supplies last until early May). Seeds may be started indoors (early February) to produce transplants, but will take 6-8 weeks before reaching transplant ready size.  Dig a trench 2-3 inches deep and plant onion transplants 3-4 inches apart in rows 8-16 inches apart covering the seedlings up to where the bulblet ends and the leaves begin.  Water the transplants with Kangaroots root stimulator to minimize transplant shock and encourage quick root development.  Use the Kangaroots every time you water for the first 3-4 waterings for best results.  Be sure to space the plants properly, as too close of planting will limit bulb size.  If you want to use some of the plants for green onions, pull up every other plant as they grow and use the immature onion as green onions, leaving the remaining bulbs spaced at 6-8 inches apart.  To sow seeds directly into the garden, lightly rake the area to be planted, sprinkle seeds approximately 1/2-1 inch apart, and cover them with 1/4 inch of peat moss, coconut coir, or a light potting soil.  As the seedlings mature, use the same rules for thinning and spacing


Long Day onions grow north of the Mason-Dixon line, as the summer days are longer than in the south.  The best varieties for growing in Cache Valley include: Big Daddy, Big Mac Hybrid, Walla Walla and Red Burgermaster.  Intermediate Day onions will grow in both long and short day conditions: we highly recommend Candy, Dulce Grande, and Super Star.  The longest storage variety we carry (only available in plants) is Copra, it keeps 8-12 months when stored properly.


Water onions deeply and frequently while trying to maintain even soil moisture, especially since they have a fairly shallow root system.  Onions need consistent watering for high quality and quantity production.  Use a soaker hose for uniform water distribution and water lightly every 5-7 days to maintain soil moisture during these essential growth stages.  A light compost mulch can help as well if daytime temperatures start to climb over 80 degrees.  


Apply ½ cup per 10 feet of row of “That’s All it Takes” complete fertilizer or Happy Frog Organic Tomato & Vegetable Food 4 weeks after transplanting or thinning to encourage vigorous plant growth.  Place the fertilizer evenly just to the side of the plants in the row and work it lightly into the soil before watering.  Make sure to water it in!   We also recommend treating your leek seed or plants with beneficial microbes and mycorrhizae (Kangaroots or Myke).  These added helpers bring nutrients and water directly to the plants that host them, making them stronger, more resistant to insects and diseases, and more drought tolerant.  Your bulbs will grow bigger and more delicious than ever before!


Keep weeds to a minimum with diligence and persistence. Treflan and Corn Gluten weed preventative herbicides are also very effective ways to stop weeds before they start, saving you hours and hours of weeding, and they are completely safe to use on onions.  Be sure to control weeds when they are small to ensure damage is not done to tender, small root systems when weeds are removed. Practice crop rotations to discourage pest problems.  Onions do not have many pest problems, but are still susceptible to thrips, onion maggots, and wireworms.  Hi-Yield Multi-Use dust or Ferti-lome Broad Spectrum Insecticide work well for all the common insect problems.  The most common diseases seen in Utah are mildew, pink root, and stem rot.  General purpose fungicides like Natural Guard Copper Soap or Ferti-lome F-stop will control most common diseases to affect onions.


Onions all vary in harvest times. Green onions can be cut as early as 50 days.  Bulb onions generally take 100-120 days to mature. Leave bulbs in the ground until the green tops fall over, generally mid August, and if they don’t start dying out on their own, you can gently push the tops over to break them, which will stop their growth and prepare them for fall & winter storage. Leave them in the ground for 1-2 weeks after this to start the drying process. When bulbs are ready to harvest, dig up the onions and leave them in the garden, roots up, for a day or two to dry up the root system. Then put them in a warm, dry location for 2-3 weeks.  After drying, the skins will be papery and thin with a dry root system.  Trim the tops and gather up the onions in mesh bags and store them in a cool area (38-45 degrees) to keep the longest. 

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1 comment

This guide was a very good reminder of how to get the most from my onion seedlings

Dale Mortensen

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