March is the Month for Planting Sweet Onions

Gordon Johnson, Extension Vegetable & Fruit Specialist;

There has been increased demand for sweet onions grown both for local sales and wholesale markets in our region. Sweet onions can be grown in Delaware but are challenging and require attention to some critical details to produce economically.

Sweet onions have low pungency which is determined by measuring pyruvate and must have a score of 5.0 µmol/gfw or less to using a standard onion pungency test to be marketed as a sweet onion in wholesale channels. Soluble solids (a measure of sugars) in sweet onions varies considerably by variety and the sweetest types will be over 7% soluble solids.

Planting date is very important to have the highest yields and largest bulbs. For sweet onions large (Jumbo) and colossal sizes greater than 3 inches in diameter have the most value in wholesale markets. Delaware trials have shown that to achieve these sizes consistently, it is necessary to plant by the end of March.

Local trials have also shown that to consistently achieve these sizes, transplants must be used and they must be grown on black plastic mulch with drip irrigation. Four foot wide plastic is laid on a raised bed so that there is a 3 foot bed top with 2 drip tapes so that 4 rows of onions can be planted 8-10 inches between rows and 4-6 inches between transplants and a drip tape between pairs of rows. Transplants are set by hand, which requires considerable labor.

Growers can produce their own transplants but they must be seeded in the greenhouse in January. Transplants are started in 200-288 cell flats at least 10 weeks before intended transplant date. Our research has shown that very small plugs (400-500 cell trays) will also produce transplants that yield a high percentage of large bulbs. 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 2016, some transplant growers do produce surplus for sale.

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’. ‘Expression’ has replaced ‘Candy’ to a large extent because it is more disease tolerant and stores better. Other yellow intermediate day varieties that showed good adaptation to Delaware it 2014 and 2015 trials included ‘Great Western’ (March planted only), ‘Bradley’, ‘Cimarron’, ‘Scout’, ‘Avalon’, and ‘Spanish Medallion’. White varieties that did well over 2 years of trials in were ‘Mt. Whitney’, ‘SV4058NU’, and ‘Solstice’. All of these varieties have the potential to produce a large percentage of large and jumbo onions, have yield potential of over 20 T/a, and can be sold into sweet onion wholesale fresh markets. Red sweet onions tested in Delaware performed very poorly and no red varieties are recommended at this time.

One versatile yellow onion that has dual purpose as an overwintering type as well as a spring transplanted sweet onion has been ‘Bridger’.

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 & K based on a soil test. An additional 25-50 lbs N/acre is applied through drip before bulbing starts.

Pennsylvania has developed a program to grow, sell, and promote sweet onions from their state. Sweet onion production in Delaware has more risk due to our hotter and more variable late spring and summer weather. However, there is still potential to develop wholesale markets for onions produced here with careful variety selection and attention to detail in production.


Onions from the 2015 variety trial in Delaware.