Growing Short-Day Onions in Arizona: Plant This Fall, Harvest Sweet Bulbs in Spring
Onions look simple on paper. Plant the wrong type, though, and you get tall green tops with no bulb at all. Plant the right type at the right time and you pull up big, sweet, juicy bulbs in late spring that taste nothing like the bag at the grocery store. The good news for Arizona gardeners is that our mild winters are perfect for the sweetest onions there are.
In this guide I cover day length, the best short-day varieties, planting in October and November, winter care, bulbing, and how to harvest, cure, store and cook your crop.
Why Day Length Matters for Onions
Onions decide when to make a bulb based on how many hours of daylight they get. Gardeners and seed catalogs sort them into three groups:
Short-day onions start bulbing when days reach roughly 10 to 12 hours of light. These are the onions for the low desert.
Intermediate-day onions (sometimes called day-neutral) bulb at roughly 12 to 14 hours. They can work in middle elevations and in some low desert gardens.
Long-day onions need 14 hours or more. They are bred for northern states and Canada.
So the first rule of onions in the low desert is simple: grow short-day types. If a packet or bundle does not say short-day or intermediate-day, I leave it on the rack.
Best Short-Day Onion Varieties for Arizona
Texas Early Grano (often sold as 502 or Texas Grano) is a big, sweet, mild yellow onion and one of the most dependable choices for desert gardens.
Yellow Granex is the type behind the famous sweet onions of the Southeast. It makes flattened, very mild bulbs.
Red Creole is a smaller, pungent red onion with good flavor and better storage life than most sweet types.
White Bermuda (Crystal Wax) is a mild, flat white onion that is lovely sliced raw.
Texas Legend and other newer short-day hybrids are bred for uniform bulbs and good resistance to bolting.
At middle elevations like Prescott, Payson or Sierra Vista, intermediate-day onions such as Candy are worth a try.
When to Plant Onions in Arizona
Low desert (Phoenix, Tucson, Yuma and areas below about 3,000 feet). October and November are prime time. You can sow seeds directly from about late September through November, or set out transplants from October through early January. If it is late October as you read this, you are right on time.
High country (Flagstaff, Show Low, the White Mountains). Plant transplants or sets in spring once the soil thaws and hard freezes ease up. Here, the longer summer days of the north let intermediate and some long-day onions bulb well.
From fall planting in the low desert, expect bulbs ready to harvest from about May into June.
Seeds, Transplants or Sets?
You have three ways to start onions, and each has its place.
Seeds are the cheapest and give you the widest choice of varieties. Onion seed does not stay viable long, so buy fresh seed each year. Sow it a quarter inch deep, either directly in the bed or in trays. If you like starting seeds indoors or on a bench, my guide to https://www.greenthumbhobbies.com/post/diy-soil-blockers shows a no-pot method that works well for onions, since you can sow a small pinch in each block and plant the whole clump.
Transplants are bundles of young onion plants, usually sold in fall at nurseries and farm stores. They are the easiest way to get a good crop because they already have a head start.
Sets are tiny dried bulbs. They are quick and easy to plant, but there is a catch. Most sets sold at big box stores are long-day types grown for northern gardens. They will often give you green onions in Arizona but not big bulbs. If you use sets, check that they are a short-day variety, or plan to harvest them young as green onions.
One more tip about size: bigger is not always better. If a transplant or set is thicker than a pencil when it goes through a cold spell, it is more likely to bolt, or send up a flower stalk, in spring. Onions that bolt make poor bulbs that do not store. Pencil-thin is the sweet spot.
Preparing the Bed
Onions have shallow roots and hate competing with weeds, so start with a loose, weed-free bed.
Sun. Give onions full sun. In our winter, they need every bit of light they can get.
Soil. Work two to three inches of finished compost into the top six to eight inches. Our native soils are alkaline and low in organic matter, and onions love a fine, crumbly bed. If you make your own compost, a https://www.greenthumbhobbies.com/post/diy-compost-sifter helps you get a fine texture that small onion roots can push through easily.
Fertilizer at planting. Mix in a balanced or phosphorus-rich fertilizer according to the label. Phosphorus helps young roots get established.
How to Plant
For transplants, trim the tops back to about four to six inches and trim the roots to about an inch if they are long and tangled. This feels harsh, but it helps the plant settle in. Plant them about one inch deep, just deep enough to hold them upright. Do not bury the base of the plant too deep, because onions bulb at the soil surface.
Space plants about four inches apart for medium bulbs or five to six inches apart for the biggest bulbs. Rows can be about 12 inches apart, or you can plant in a grid in a raised bed.
For seeds, sow thinly in rows and thin the seedlings to their final spacing once they are a few inches tall. The thinnings are delicious, so nothing goes to waste.
Watering Through Winter
Onions need steady moisture to make big bulbs. Uneven watering can lead to split or small bulbs.
In the low desert, plan on keeping the top few inches of soil evenly moist. In cool winter months that might mean watering once or twice a week, depending on rain, wind and your soil. As days warm up in March and April and the bulbs start to swell, they will need more water, often two or three times a week. Drip line or a soaker hose works well because it keeps the leaves dry and puts water right at the shallow roots.
Feeding for Big Bulbs
The size of an onion bulb depends on how many healthy leaves the plant grows before bulbing starts. Each leaf corresponds to a ring in the bulb, so more leaves means a bigger onion. That is why feeding in winter matters.
Side-dress with a nitrogen fertilizer every two to three weeks from about a month after planting until bulbing begins, following the label. Water it in well. Once you see bulbs start to swell and the necks begin to soften, usually in early spring, stop adding nitrogen. Feeding too late can lead to thick necks that do not cure or store well.
Winter Care, Frost and Pests
Onions handle frost well. A light freeze will not hurt established plants. If a hard freeze is in the forecast, frost cloth over simple hoops gives them extra protection. My guide to https://www.greenthumbhobbies.com/post/diy-row-cover-hoops shows an easy way to build them.
The main pest to watch for is onion thrips. These tiny insects feed on the leaves and leave silvery streaks. A strong spray of water helps knock them back, and insecticidal soap can help when used according to the label. Healthy, well-watered plants handle a few thrips just fine.
If a plant sends up a flower stalk, pull it and use it soon as a green onion or in the kitchen. It will not make a good storage bulb.
What Bulbing Looks Like
In late winter and early spring the base of each plant starts to widen, then the bulb swells and pushes up out of the soil. That is normal. Do not pile soil back over the bulbs. Onions like their shoulders above the ground.
As the bulbs mature, the necks soften and the tops start to flop over on their own. This is your signal that harvest is close.
When and How to Harvest
In the low desert, most fall-planted short-day onions are ready from about May into June. Watch the tops. When about half to three-quarters of the tops have fallen over, stop watering and let the bed dry for about a week.
Loosen the soil with a garden fork and lift the bulbs instead of yanking the tops, since bruised onions keep poorly. Brush off soil, but do not wash them.
Curing Onions
Curing dries the outer skins and seals the neck so the onions store well. It is easy.
Lay the onions in a single layer in a shady, dry, airy spot. A covered patio or a screen on a couple of bricks works well. Our late spring sun can scald onions, so shade matters.
Leave them for about two to three weeks, until the necks are completely dry and the outer skins are papery. Then trim the tops to about an inch and trim the roots.
Storing Your Onions
Here is something that surprises a lot of gardeners: the sweetest short-day onions do not store as long as the strong, pungent storage onions from the north. Their high water and sugar content is what makes them so mild, and it also makes them spoil sooner.
Sweet types like Grano and Granex keep for about one to two months in a cool, dry place with good airflow.
Red Creole and other more pungent onions usually keep longer.
Store them in mesh bags, baskets or the classic trick of an old pair of pantyhose with a knot between each onion. Cut one off as you need it.
For longer keeping, chop and freeze sweet onions, or slice them and dry them.
Cooking With Homegrown Sweet Onions
This is the best part. Sweet short-day onions are mild enough to eat raw.
Raw on everything. Slice them thin for burgers, sandwiches, salads and tacos.
Caramelized onions. Slow-cook sliced onions in butter or oil over low heat for 30 to 45 minutes until deep golden. Freeze them in small portions for soups, pizza and eggs.
Quick pickled red onions. Pack thin slices in a jar, cover with a hot mix of vinegar, water, a little sugar and salt, and refrigerate. They are ready in about an hour.
Green onions. Use young thinnings in stir-fries, salsa and on baked potatoes.
Pairing Onions With Other Crops
Onions fit into almost any winter bed alongside carrots, beets and lettuce, and their cousins are just as easy. My guides to https://www.greenthumbhobbies.com/post/garlic-arizona and https://www.greenthumbhobbies.com/post/leeks-arizona cover two more alliums that thrive in the same season, and https://www.greenthumbhobbies.com/post/carrots-arizona is a classic onion bed partner.
My Quick Onion Checklist
Choose short-day varieties for the low desert.
Plant pencil-thin transplants, fresh seed or short-day sets in October and November.
Space them four to six inches apart in a loose, compost-rich bed.
Keep soil evenly moist and feed with nitrogen every two to three weeks until bulbing starts.
Stop feeding when bulbs swell, and stop watering when most tops fall over.
Cure in the shade for two to three weeks, then store cool, dry and airy.
Plant a bundle this week, and by late spring you will be pulling up sweet bulbs. Happy planting!
Comments