Immense quantities of onions are bunched when the tops are green and sold from early spring until midsummer. In the South, white and yellow multipliers are used; in the North, sets grown from seed and also those of the Egyptian tree onion.
Multipliers are generally planted in the fall, about six weeks before freezing weather. The trenches should be 4 or 5 inches deep and the bulbs set 3 to 6 inches apart. Large bulbs of the potato class, planted in the spring, will produce a great many small bulbs for planting in the fall of the same season. They will make a good start in the fall, grow to some extent during the winter in mild localities and make rapid progress in the spring. When multipliers are planted where the winters are severe, a mulch of some kind, preferably strawy manure, should be applied after the ground is frozen.
In the North, enormous quantities of sets are planted to produce bunching onions for local markets. It is customary to plant the sets 1 to 2 inches apart with I foot of space between rows, as soon as the ground can be prepared in the spring.
Because of its hardiness the Egyptian tree onion (top onion) is a favorite throughout the North. For the earliest bunching onions, the sets should be planted in the fall at least six weeks before freezing weather, and mulched if possible with manure, after the ground is frozen.
Considerable quantities of bunching onions are also grown from seed sown in the open ground, for marketing during the summer. This method requires the free use of seed, 20 to 30 pounds an acre. It is an economical method of production, and good profits are possible when prices are materially lower than for bunching onions grown from sets.
Bunching onions are sometimes prepared for market in the field (Figure 51), but it is better to take them to the packing shed, where the dead leaves can be removed and the onions properly washed and bunched. From 4 to 10 onions are tied in a bunch, the number depending upon size and market requirements.
The imported onion maggot (Pegomya cepctorum) is closely related to the cabbage maggot (363). The carbolic acid emulsion treatment, previously described, is probably the most effective application. Rotation, however, is the most certain means of avoiding loss from this enemy as well as from all other insect and fungous pests of the onion.
The onion thrip (Thrips tabaci) is often a destructive enemy, especially in the South and in the Southwest, where Bermuda onions are grown. It is very minute in size, not exceeding 1-20 inch in length, and provided with sucking mouth parts and bristle-like mandibles. When present in large numbers thrips cause the plants to turn brown and die. Spraying with kerosene emulsion is considered the most successful treatment. Bordeaux also serves as a repellent.
Onion smut is the most serious disease of the onion. It is likely to become particularly troublesome when rotation is not practiced. After the land is once infested, this fungus is exceedingly difficult to eradicate. The entire plant may be attacked, and the spores, forming a black dust, are readily disseminated by the wind, insects, and tillage implements. As the seed may also transmit the disease, it is sometimes soaked for about 20 minutes in a solution of formalin (1 ounce in 1 gallon of water), and thoroughly dried before being drilled. No method of soil or plant treatment has been found fully satisfactory.
Downy mildew (Peronospora schleideniana), which sometimes appears in warm, sultry weather, causes the leaves to blight. The spores present a downy, violet covering. Timely applications of bordeaux will control this disease.