Plastic mulch is becoming more popular to use when growing a variety of vegetables, including onions. In fact, it is becoming so popular for weed control, water conservation and to regulate soil temperature, there are many different types of plastic mulch available for the farmer and gardener. For example, white plastic mulch is used to keep soil cooler in summer and black plastic mulch is used to keep the soil warmer in early spring and winter.
Choose onion sets or plants that are adapted to your area. Onions are separated into long day, intermediate and short day varieties. In the Southern U.S., for example, short day varieties are planted in early spring because by the time day length increases for a long day variety, the daytime temperatures are too warm to grow onions.
Prepare the planting area by removing weeds and debris and creating a smooth area or slightly raised rows. Creating six-inch high hills keeps you from bending over as far while planting numerous onion sets.
Lay drip tape or drip hoses along the length of the hills for irrigation. This is very important because you cannot get water to the root systems of the onions once they are surrounded by the plastic mulch. Onions grow best in evenly moist soil in a well drained location. Rows should be 24 to 36 inches apart.
Cover the planting area with the plastic mulch of the desired colour. Secure the plastic by covering the edges with rocks, boards or wire pins pushed through the plastic and into the soil every 18 to 24 inches.
Cut "X" shaped places in the mulch with a sharp knife or other cutting tool 6 inches apart along the row. Be careful not to cut the drip tape or soaker hose while cutting the mulch. Each "X" should be 4 inches wide and be close enough to the irrigation hose the onion plant can receive water. Plant the onion plants, or sets, 1 inch deep in each area where the "X" is cut. The onions will grow under the mulch and the onion foliage will grow above the mulch.