Detailed Explanation of Key Technical Points for Overwintering Onion Seedlings by Heeling In

Before heeling in, first dig the seedlings out of the field (try not to damage the root system), shake them gently to shake off loose soil, and remove seedlings with no roots, no growing point, overly short, or thin and spindly ones, as well as leggy seedlings with overly long leaves, tillering seedlings, and pest- or disease-damaged seedlings, along with scallion seedlings (the leaf sheath part of scallion seedlings grows upright; the cross-section of the leaf base is round: the leaf sheath part of onion seedlings is upright and slightly curved, and the cross-section of the leaf base is concave). For ease of management, onion seedlings should be heeled in by grade. The grading standards are: grade-one seedlings, small bulbs 0.6 to 0.7 cm, 3 to 4 leaves; grade-two seedlings, small bulbs 0.4 to 0.6 cm, 3 leaves. While grading, tie them into small bundles of 80 to 100 plants, and after tying, heel them in by grade.

Grade-one seedlings are heeled in inside a sunny cold frame. The sunny cold frame should be chosen behind a windbreak or in another sheltered and shaded place. First spread a layer of dry sandy soil about 5 cm thick in the frame, then pile a small ridge 5 to 7 cm high on the dry sandy soil on one side of the frame, and behind the ridge place grade-one seedlings bundle by bundle in a row side by side. After one row is placed, bank soil 5 to 7 cm around the roots of the seedlings, then place straw with a diameter of 2 to 3 cm on the banked soil to separate the front and back rows of seedlings. After all are heeled in, seal the surroundings of the frame tightly with soil and firm it to prevent wind and water from getting through.

Grade-two seedlings are heeled in inside a cold-protection trench. To prevent root rot, the cold-protection trench should be chosen on higher ground. The specific method is: open several trenches east-west at intervals of 5 to 10 cm, with trench length 3 to 5 m, width 25 to 30 cm, and depth 15 to 20 cm. Place the bundled seedlings in the trench tilted 45 degrees toward the south side, and bank soil up to the top of the leaf sheaths. After all are heeled in, set up windbreaks around them so the seedlings are sheltered from wind and shade, and seal the outside of the windbreaks tightly with soil to prevent wind and rain. Seedlings heeled in by this method will still have their central leaves grow slowly. In the Northeast region, as the outside temperature continues to drop, soil should be added again in batches or crop straw should be used for cold protection according to weather conditions.

Pay attention to weather forecasts at ordinary times. Before cloudy or snowy weather, cover the seedlings with plastic film; after snow, remove the film promptly and sweep away surrounding snow to prevent snowmelt from entering the frame or trench and causing seedling rot.

By applying the above methods, onion seedlings can safely overwinter until they are planted out in the field in early March of the following year.