How to Plant Onion Transplants
One inch of each transplant should be covered with soil, along with its roots, when you plant onion transplants, according to the University of Kentucky Cooperative Extension. Set them out about four weeks before your area’s average last frost date, rather than waiting for reliably warm weather. Good soil preparation, correct spacing, and the right day-length variety matter as much as the planting hole.
Use these measurements while planning and planting the bed.
| At-Planting Detail | Target |
|---|---|
| Outdoor planting time | About 4 weeks before the local last frost |
| Soil covering each transplant | Roots plus 1 inch of the plant |
| Spacing within a row | 2–4 inches apart |
| Space between rows | 12–18 inches apart |
| Bed soil pH | 6.5–6.8 |
| Ready-to-plant transplant | About 6 inches tall and roughly half the thickness of a lead pencil |
Choose Onion Transplants That Suit Your Garden
Onion transplants are young onion plants grown from seed, usually sold as small bare-root bunches or in cell packs. They are different from onion sets, which are small immature bulbs grown the previous year and stored over winter.
For bulb onions, choose a variety suited to your latitude and day length. Short-day, intermediate-day, and long-day onions begin bulbing in response to changing daylight hours. A healthy transplant of the wrong day-length type may make only a small bulb, even in excellent soil.
Transplants are especially useful for storage onions because they give the crop a head start over direct seeding. Look for plants with firm bases, healthy roots, and green leaves. A shipment of bare-root plants may look dry after travel, but that alone does not mean the plants have failed.
How to Plant Onion Transplants
Plant onion transplants in a fertile, well-drained bed about four weeks before your local average last frost.
Step 1: Pick the Planting Date and Harden Indoor Seedlings
Plant outdoors about four weeks before your local average last frost. Onion transplants tolerate light frost, so use the local frost date as your guide instead of waiting for summer-like temperatures.
For the strongest start, prepare the bed first and move hardened seedlings outdoors.
If you are raising your own starts, sow onion seed indoors about 8–10 weeks before outdoor planting. Aim to transplant plants when they are about 6 inches tall and about half as thick as a lead pencil.
Indoor-grown seedlings need a gradual transition to sun, wind, and outdoor temperatures. If seedlings grow taller than 5 inches before hardening off, trim the leaves back to 3 inches. Expose them to outdoor conditions a little at a time; they are generally ready for the garden after about a week of gradual exposure.
Light frost is usually harmless, but avoid setting out newly arrived or poorly hardened plants just before a prolonged spell below 45°F. Extended cold exposure can encourage some onions to bolt, sending up a flower stalk instead of producing a good bulb.
Step 2: Prepare a Fertile, Well-Drained Bed
Onions need loose, well-drained soil because their roots are shallow and relatively fine. They grow best in sandy or loamy soil with moderate to high organic matter and a nearly neutral pH of 6.5–6.8.
If you can, test the soil before planting. Without a soil test, University of Connecticut Home & Garden Education recommends incorporating 2½ to 3 pounds of 10-10-10 fertilizer, or an equivalent organic fertilizer, for every 100 square feet of garden space before planting.
For a 100-square-foot bed, also work in 3 to 4 bushels of leaf-based compost, peat moss, or bagged humus. Mix amendments through the planting area instead of leaving concentrated pockets of fertilizer beneath individual plants.
Break up clods, remove large stones, and level the bed. Do not plant in a low area that stays wet after rain or irrigation; onions can rot in poorly drained soil.
Step 3: Mark Rows and Set the Spacing
Lay out rows 12–18 inches apart. This gives you room to weed, water at the soil line, and inspect plants without stepping near their roots.
Space bulb-onion transplants 2–4 inches apart within each row. Use closer spacing if you want smaller bulbs or plan to pull some plants as green onions; use the wider end of the range for plants you intend to leave for full bulbs.
I use a stirrup hoe to draw the row line because it makes a narrow, shallow furrow without turning over the whole bed.
Step 4: Set Each Transplant at the Right Depth
Separate bare-root plants gently so you do not tear the roots. If roots are tangled, tease them apart with your fingers rather than pulling the bunch apart quickly.
Make a small hole or shallow furrow, arrange the roots downward, and cover the roots plus 1 inch of the transplant with soil. Firm the soil lightly around the plant so it stands upright, but do not compact the bed.
Dry-looking bare-root transplants can go straight into the garden.
Step 5: Water In and Finish the Planting
Water the row thoroughly after planting so the soil settles around the roots. The goal is moist soil around the root zone, not a puddled or saturated bed.
Label each row with the onion variety and planting date. This matters because different varieties bulb at different times, and it helps you track which onions perform best in your garden.
Before leaving the bed, check that plants are upright, roots are covered, rows drain freely, and no transplant has been buried so deeply that its leaves are covered with soil.
Handle Common Onion Transplant Situations
Dry Bare-Root Bunches
Bare-root onion transplants commonly arrive dry and flattened. Plant them promptly in prepared soil, water them in, and give them a few days to resume growth.
Dense Seedling Flats
If seedlings were grown closely together in a flat, lift a small section with a fork or plant label and separate plants slowly. Hold seedlings by the leaves rather than squeezing or pinching the small stem at the base.
Keep roots shaded while you work. Planting a few at a time prevents exposed roots from drying in sun and wind.
Raised Beds and Heavy Rain
Raised beds work well for onions because they can improve drainage, especially where rain is frequent or soil contains a lot of clay.
After a hard rain, inspect the row for washed-out roots or plants leaning sideways. Reset any loose transplant while the soil is still moist and easy to work.
Care for Onion Transplants After Planting
Water near the soil line rather than spraying foliage whenever practical, and water early enough in the day for leaves to dry.
Weed early and shallowly. Onion leaves do not shade weeds well, and weeds compete strongly for moisture and nutrients. A light layer of clean organic mulch can help conserve soil moisture and reduce splashing, but keep mulch from pressing against the plant bases.
Onions are heavy feeders, so a well-prepared bed is important. If plants remain pale, stunted, or weak despite proper moisture and weed control, a soil test is a better next step than adding fertilizer repeatedly without knowing what the soil lacks.
For green onions, pull individual plants whenever they reach a useful size. For mature bulb onions, allow the bulbs to enlarge without piling soil around them. Harvest bulb onions after the foliage yellows and falls naturally, then cure the bulbs in a dry, airy, shaded place until their outer skins and necks are dry.
Prevent Onion Diseases and Pests
Use only healthy, pathogen-free transplants or sets. This is especially important where onion white rot has occurred, since the disease can cause yellowing leaves, root decay, and rotting bulbs.
Do not plant onions, garlic, or other alliums in a bed known to have onion white rot, and do not move soil or plant debris from that area into clean beds. The white rot pathogen can survive in soil for up to 40 years.
Watch foliage as the plants grow. Tiny speckled holes, silvery feeding marks, or twisted leaves can indicate onion thrips. Poor growth, wilting, or small worms in the bulb or stalk may point to onion root maggots. Identify the problem before choosing a control method, since overwatering, nutrient problems, and pests can all cause weak growth.
Clean planting stock, weed control, and avoiding movement of infected soil do more to prevent serious problems than reacting after plants are badly damaged.
How to Tell Onion Transplants Are Established
A successful planting begins to show upright leaves and fresh green growth.
Use this quick check after planting:
- Plants are standing upright rather than lying flat.
- Soil is settled around the roots without crusting.
- Rows remain mostly free of competing weeds.
- Leaves show fresh green growth rather than continued yellowing.
- Plants are labeled by variety so you can compare bulb size and timing later.
If several plants decline together, inspect drainage first. If plants are yellowing or rotting at the base, determine the cause before changing your watering or fertilizer routine.
Mark your local last-frost date on the calendar and prepare the bed before your transplants arrive.
Frequently Asked Questions
What Cannot Be Planted Near Onions?
No common vegetable needs to be banned simply because of a companion-planting rule. The important exclusion is a bed with known onion white rot: do not plant onions, garlic, or other alliums there, and do not move soil from that spot. The pathogen can remain viable for up to 40 years.
Are Onion Sets and Onion Transplants the Same?
No. Onion transplants are young plants grown from seed, while onion sets are small bulbs that were grown, harvested, cured, and stored the previous year. Transplants are often a better choice for growing storage onions because they begin as seedlings rather than second-year bulbs.
How Do I Get Onion Seeds?
Let selected healthy onion plants complete their second-year growth and produce flower stalks. When the flower heads dry, collect the black seeds and dry them fully before storing them in a cool, dry place. Grow one variety for seed saving, or keep flowering varieties separated, to reduce unwanted cross-pollination.
How Long Do Onions Take to Mature?
Bulb onions are ready based on plant condition rather than one universal number of days. The variety’s maturity window and day-length type affect timing, and foliage that yellows and falls naturally signals that bulbs are approaching harvest. Green onions can be harvested much earlier, once the stems and small bulbs are useful in the kitchen.