As onions mature, their dry matter content and pungency increase, with a resulting increase in storage potential. Onions are ready for harvest when at least half the leaves are dead. Tops are beginning to fall in many fields. Pull the bulbs by hand or use equipment such as a potato digger or undercutter to cut the roots and lift the bulbs. If you wait until all the leaves are dead and dry, it’s likely that the outer skins will be loose rather than firm, which may not hurt the keeping quality but the onions will not look as nice. However, pulling too green will make it difficult to cure them well. Harvest when the weather is dry; harvesting after a rainfall or when the humidity is high increases susceptibility to post-harvest disease.
For optimum storage quality, onions must be cured soon after harvest. Optimum conditions are 68-86°F and 70% relative humidity for at least 12 to 24 h. Curing decreases the incidence of neck rot, reduces water loss during storage, prevents microbial infection, and is desirable for development of good scale color. Curing can be done in the field, preferably when the weather is warm and dry. If it rains, let them dry fully before handling – don’t handle the bulbs when they are wet. A greenhouse or hoophouse also provides good conditions for curing. Temperatures in the 80’s will enhance the bronze color in the skins. Sunshine is good as long as it is not too hot. Extremely hot sun with temperatures in the 90’s can produce sunscald. Onions curing on a sandy soil will get hot quicker than those lying on a heavier soil. In a greenhouse, temperatures should be held below 85 degrees F, which will probably require leaving everything wide open. Using a black shade curtain over the house can help. Curing is complete when the neck is completely dry and tight. If the neck remains open it
allows entry of pathogens such as Botrytis neck rot.
The next step is topping. Mechanical onion toppers are essential for larger plantings, and for the needs of a small diversified farm they are probably best obtained second-hand. Check your favorite used equipment dealers! Onions can also be topped by hand using clippers. Handle gently to avoid bruising. Defective onions (i.e. sprouted, insect damaged, sunscalded, green, bruised) should be discarded. Grade for size according to your markets.
To ensure maximum storage, onions must be promptly stored after curing. Get them out of the sun; exposure to light after curing will induce greening of the outer scales. The optimum temperature for long-term storage of onions is 32°F with 65-70% relative humidity, but it is important to bring them down to this temperature slowly. In fact, holding onions in a barn or garage so that they cool along with the average outdoor temperature in late summer and fall works quite well. Avoid cooling bulbs to well below the average daily temperature, because they will draw moisture from the warmer air, which can lead to disease. If you are selling them within a couple of months, keeping them in an un-insulated barn is fine. To hold longer, an insulated storage room will be needed.
Harvest Tips for Best Quality
1) Be sure onions are well dried and necks tight (i.e. the tissue does not slide when you roll your neck between your fingers) before topping. Bacterial diseases and Botrytis Neck rot can move through green tissue into the bulbs. These diseases do not move in dry tissue.
2) Leave 2-3 inches of neck on the bulb. This increases the distance from the cut surface to the bulb for these pathogens to travel.
3) Minimize mechanical injury during harvest & topping. Reduce drops to 6” and pad sharp surfaces. Bruises provide direct entry points for diseases to get started.
4) Grade out damaged onions before putting them into storage. Damaged bulbs give off moisture, which is favorable for development of diseases in storage.
- John Howell, Andrew Cavanagh, & Ruth Hazzard. Resources: CSU Extension and the University of Saskatchewan.