Heat Unit Accumulation in Onions
January is a normal time for our customers in the South to reach out, worried that their plants from November have not started to bulb yet. Our response, “They are on the right track. No need to be concerned.”
Here’s a bit of background on the onion growing cycle.
Once you plant your onion plants, there is a roughly 2-week period where they are transitioning into their new environment and establishing a new root system. Some may know this as starting their “second life cycle”.
New Leaf Generation
All plants require a certain number of heat units to grow. Onions grow new leaves based on accumulated heat units. After the new root system has been generated, the plant will begin to take up those heat units. It takes 160-200 heat units for an onion to shoot a new leaf; this number is dependent on variety with short day onions generally requiring 160 and long day varieties requiring 200. The basis for onions is 45 degrees so each day the average temperature is above 45 degrees, the onion acquires that many heat units for that day. For example, if your high was 70 and your low was 50, the onions would acquire 15 heat units that particular day. This is determined from (70+50)/2-45=15. The process then repeats itself until bulbing begins.
The Carbohydrate Transfer
The bulb of the onion is formed from the transfer of carbohydrates from the leaves to the bulb of the plant. For the bulbing process to begin, the plant must accumulate a total of approximately 2200 heat units. When the right combination of daylength and total heat units are acquired, the onion begins the transfer of carbohydrates from the leaves to the rings of the bulb. Ideally, the heat units and daylength are met at the same time to help ensure maximum size potential for that onion.
Daylength in Onions
Short day varieties require 10-12 hours of daylength while long day varieties require 14-16 hours of daylength. This is why we stress planting your onion plants at the proper time; it is crucial for these two events to happen as close together as possible. Planting too early could result in frozen plants and planting too late will result in bulbs not reaching their full-size potential.
Once all those carbohydrates are transferred, the top can no longer support the weight of the leaves. This causes the top to fall over. Think of a drip hose that shrinks after you turn the water off. It finally crimps and no more water comes out.
As always, if you have any questions regarding your onion plants, please don’t hesitate to contact Customer Service email@example.com!
Is it good to prune onions if so at what stage should you prune.
Normally onions are not pruned. An onions whole life cycle is about growing leaves and then growing the food storage unit, the part that we refer to as the onion and the part that we eat. From an onions point of view the bulb is what will feed the plant next year when it goes to seed, it’s way of ensuring the survival of the onion. If one was to prune the onion this would not tell it to make more leaves, it would simply take away part of it’s food producing material, the leaves.
I’m at latitude 35.817, near the Smoky Mtns. Nat’l. Park.
I don’t get sunlight until about 9:00AM, and it gets blocked
by the mountains around 4:00PM (or a little earlier in some places).
We get lots of 30 degree temperature changes from day to night…so
lots of freeze/thaws. Technically, we are a zone 6b/7a, although we only
get a couple (or a few) lows into the teens. We get lots of rain.
What onions should I plant, and when?
Onions can withstand light freezes down to 20 degrees for “short” periods of time; not entire days. Now is the time for your area to plant. As far as the sunlight goes, take the days to harvest and add on to those as your onions will need additional time to acquire heat units due to the the location. Intermediate day varieties should work best for you.