Onion Planting Guide


When planting onions choose a garden site that is in full sun and has a well-drained soil. A soil pH of 6.0 to 6.5 is ideal for growing onions. Apply lime and fertilize according to soil test results. See Tracking and Adjusting Soil pH for information on adjusting the pH range of Garden soil.

Many gardeners plant onions in a raised bed, rich in well-rotted Compost, manure, and other organic matter. Manure that contains straw or wood shavings may cause a temporary lack of nitrogen in the soil. The bacteria breaking down organic waste utilizes nitrogen and takes it from the soil. The plants may suffer from this, so add nitrogen fertilizer when digging such manure into the soil. Onions can be grown from seeds, small dormant onions called “sets,” or onion transplants.

Starting From Seed

Onion Seeds can be sown in the garden in the midfall to over winter and produce a crop next season. Planting them directly in the soil in the spring is only advisable in the best of climates otherwise they should be seeded into cell pots or a suitable container for transplant into the garden in spring.

Sow onion seeds about 1/4 inch deep, covering lightly. Most onion seed varieties should germinate in approximately 2 weeks. Grow onions in individual peat pots or cell packs to reduce transplant shock, several plants per cell or pack are advisable. A relatively cool location with full sun for optimal results.

Transfer onion transplants into the garden after the danger of frost has passed. A hardening off period is advisable. Place your seed trays outdoors for a few hours each day. Increase their time outside every day for about a week.After slightly more than a week of gradual exposure to the outdoor climate, your plants will be ‘hardened’ enough to withstand the shock of transplant.

Onion sets can be planted directly into the ground in very early spring, as soon as the soil is workable. Spacing and depth varies among varieties but common sense dictates that you anticipate the full size of the onion plant, whatever variety you are planting, and leave adequate space for the bulbs to develop. If you are a tad off the optimal spacing – beware the sky is gonna come crashing down and the Universe is now out of synch.


When growing onions keep in mind that they are are shallow rooted and any cultivation should be done carefully to avoid damage to the bulbs or roots. Cultivation should be shallow, without bringing excessive soil to the plants. Many gardeners pull the soil away from plants in order to allow greater bulb expansion. However, this practice is not needed on well prepared, highly organic soil. Mulching can also help control weeds.

Growing Onions in well-drained soils will discourage disease problems. It is always wise to follow a rotation schedule when planting onions.


Watering is critical. Water slowly and deeply to produce healthy onions. They should receive about an inch of water per week slightly more in sandy soils. Inexpensive Water Timers are available . .

Always water immediately after feeding, as harvest time approaches the onion will require more water.


  1. Apply a modest amount of a complete fertilizer such as 8-8-8 or 10-10-10, a few days prior to planting. 
  2. Onions are heavy nitrogen feeders a nitrogen-based fertilizer such as Ammonium Sulfate or Ammonium Nitrate should be applied at the rate of one cup per twenty feet of row , applications should continue at the same rate every 18 – 20 days {Or as per label instructions}. Excessive applications of nitrogen can delay maturity of bulbs, cause thick necks and splitting.
  3. About 4 weeks before harvesting discontinue fertilization. Organic gardeners should incorporate a nitrogen rich compost into the soil.  Always water immediately after feeding, as harvest time approaches the onion will require more water.

    Also See:Understanding Fertilizer Labels

Weed Control

When growing onions keep in mind that they are are the least suited to compete with weeds. Weed pressure can be especially damaging to young onion plants because they are slow growing, have shallow roots and do not have enough foliage to adequately shade the ground. Commercial growers use products such as DACTHAL and BUCTRIL, these are not readily available to Home gardeners and not really necessary. Herbicides are not recommended , as many attack the crop also. Careful, shallow cultivation and weeding is essential.

Companion Planting

Good Neighbors of Onions are Beets, Cabbage, Carrots, Celery, Cucumber, Lettuce, Pepper, Squash, Strawberries, and Tomato . Bad Neighbors include Bush Beans, Pole Beans, Peas.

Do not plant onions or related crops – other members of the Amaryllidaceae family – in the same soil for more than one season.. Rotate the crops to various sections of your garden space from season to season.

Planting Chamomile with onions. Improves growth and flavor

See – Companion Planting

Harvesting and Storage

Onions will store better if they are dried for several days outdoors, away from direct sunlight.

  • Leave the tops on the bulbs during drying.
  • After drying, cut tops within an inch of bulb.
  • Fresh sweet onions can be stored for several weeks in a cool, dark place. They can be stored in the refrigerator, but do not put them in plastic bags. This will inhibit air circulation.
  • Storage onions should be dried for a longer period of 10 to 14 days. After cured, the tops can be removed and onions stored in mesh bags, or dried tops can be braided into a string of onions.