Success with Sweet Onions

Gordon Johnson, Extension Vegetable & Fruit Specialist;

There has been increased demand for sweet onions grown both for local sales and wholesale markets. Success with sweet onions starts with growing or obtaining transplants of adapted varieties. Direct seeded bulb onions do not perform well in spring plantings. Transplants are started in January in 200-288 cell flats in the greenhouse for March transplanting. Growers can also arrange to have transplant growers in the Southwest (Texas, Arizona) produce transplants and ship them to our area for spring planting. While it is too late to have plants grown for 2023, some transplant growers do produce surplus for sale.

Onions also benefit greatly by being transplanted into black plastic mulch. Four-foot-wide plastic is laid on a raised bed such that there is a 3-foot bed top with 2 drip tapes. Four rows of onions can then be planted in the bed, with 8-10 inches between rows and 4-6 inches between transplants and a drip tape between each pair of rows.

Sweet onions are transplanted into black plastic mulch using four rows with 2 drip irrigation lines.

Sweet onions are transplanted into black plastic mulch using four rows with 2 drip irrigation lines.

Planting date impacts yield and bulb size, so it is very important to transplant at the right time. For sweet onions large (Jumbo) and colossal sizes greater than 3 inches in diameter have the most value. To achieve these sizes consistently, it is necessary to plant by the end of March on Delmarva. The later you plant in April, the lower the yields and sizes obtained.

Spanish Medallion sweet onion near harvest.

Spanish Medallion sweet onion near harvest.

Sweet onions have low pungency which is determined by measuring pyruvate and must have a score of 5.0 µmol/gfw or less, using a standard onion pungency test, to be marketed as a sweet onion. Sulfur should be avoided in fertilizers for sweet onions to limit pungency.

In general, intermediate day sweet Spanish onion types are best adapted for our area; however, some long day varieties also can be grown successfully. The standard yellow sweet onion variety has been Candy. Other recommended yellow varieties are Spanish Medallion, Expression, Great Western, and Highlander. White onion varieties recommended include Great White and Sierra Blanca.  No red varieties are recommended at this time.

It is important that once transplanted, onion growth is not interrupted. Steady, frequent applications of irrigation water are necessary because onions have small root systems. If beds are allowed to dry out at any time, yields will be reduced. Fertility varies with grower and field but in general 50 lbs. of N/acre is applied preplant along with P and K based on a soil test. An additional 25-50 lbs. N/acre is applied through drip before bulbing starts.

Allium Leafminer is a new pest to the mid-Atlantic area and needs to be controlled in onions.  It is a long grey-black fly with a distinctive yellow or orange patch on the top of its head, and yellow sides. The larvae are a typical whitish maggot. Females repeatedly puncture leaves with their ovipositor, resulting in a line of small white dots. Leaves can be wavy, curled, and distorted. Larvae mine leaves and move into bulbs and leaf sheathes where they pupate. Covering plants with light weight row covers in March-April during the adult flight can exclude the pest. Systemic and contact insecticides can be effective. However, multiple applications will be needed during the 5–7-week flight period in early spring starting at planting. Onion thrips are another common insect pest. Populations frequently increase following adjacent alfalfa or small grain harvest, as adults overwinter in these fields. Thrips pierce plant tissue and remove plant liquids. Immature thrips usually feed on young tissue between the leaf sheaths and stem, adults feed on more mature tissue. Feeding damage on leaves looks like whitish or chlorotic streaks. If feeding is severe, particularly under dry conditions, the tips of leaves become brown. Prolonged feeding reduces bulb size and increases the incidence of leaf and bulb rots. There are 3-5 overlapping generations per season. Effective management relies primarily on foliar insecticide sprays. High spray pressures and high gallonages are necessary to ensure good contact between the pest and chemical. Twin flat fan nozzles result in greater coverage than single flat fans.

Bacterial diseases such as center rot are also a problem.  For sweet onions grown on plastic mulch, consider transplanting into silver reflective, white, or black biodegradable plastic mulch to reduce the soil temperatures associated with increased losses due to center rot. When conditions are favorable for bacterial diseases, typically warm and wet, initiate a preventative program consisting of fixed copper tank mixed with mancozeb or ManKocide at 2.5 lb/A.