You can store onion varieties together in the same room or storage space but should always keep them in separate containers. Sweet onions spoil much faster than storage types do. When you mix them in the same bag or bin, the rot from spoiling sweet onions can spread to your long-keeping varieties and ruin them all.
I learned this the hard way my first year storing a big harvest of many mixed onion types. I put my sweet Walla Wallas in the same mesh bag as my pungent yellow storage onions to save space. Within a month, the sweet ones had started going soft. The mold spread to three of my storage onions before I caught the problem. I lost bulbs I could have saved with better sorting.
The difference between sweet vs storage onions comes down to their chemical makeup inside each bulb you grow. Your sweet onions have low levels of pyruvic acid which gives them their mild, gentle flavor that tastes great raw in salads. But this same chemistry makes them far more likely to break down and spoil in your storage space. The high water content doesn't help either.
Michigan State research puts clear numbers on how long each type lasts in your storage space. Sweet onions keep for just two to three weeks even under ideal conditions. Pungent storage varieties can last one to eight months. Your curing process and variety choice matter a lot. That's a massive gap in shelf life between the two types.
The mixing onion types storage problem gets worse because rot spreads so fast between your bulbs when they touch. One soft onion can infect its neighbors within days if you pack them close together in the same bin. The mold and bacteria move from the rotting bulb onto the healthy skins of your nearby onions through direct contact. Keeping different types apart stops this chain reaction in your storage area.
Good onion variety storage starts with sorting your harvest into groups right after curing is done. Put all your sweet onions in one bag on one side of your storage space. Put your storage types in another container on the other side of the room. Label each container so you know which ones to use first. This simple step saves you from losing good bulbs to cross-contamination later on.
In my experience, mesh bags work great for keeping different types apart in the same cool storage space. The bags allow air flow around each bulb which helps prevent moisture buildup on the skins. Hang multiple bags on hooks spaced apart. Even if one type starts to spoil, it won't touch the others nearby in your storage area and spread rot.
Check your sweet onions every week since they go bad so fast compared to your storage types in the same space. Give each bulb a gentle squeeze to feel for soft spots that signal the start of rot inside. Pull out any suspect bulbs right away and use them up in cooking that same day. Your storage varieties need checks only once a month since they keep so much longer in the cool.
Plan to use up all your sweet onions within the first full month after harvest each year. You can then enjoy your storage types through fall and winter. This order of eating means you work through the quick spoilers first. Save the keepers for the cold months when you need them most in the kitchen. With good sorting and regular checks, you can enjoy home grown onions for six months or more from a single harvest.
Read the full article: 7 Essential Signs for When to Harvest Onions
