DRY BULB ONION MATURITY
By
Jim Christopherson, Keithly-Williams Seeds
The following is a discussion concerning proper physiological onion maturity. This
discussion quantifies onion maturity into several stages that are recognizable to onion
growers. Much of this discussion is from careful field observations over several years
combined with grower input and discussion. Your ideas and thoughts are greatly
encouraged.
TEN STEPS TO PROPER ONION MATURITY:
Step 1: Last Leaf
To learn onion maturity we must first learn how an onion plant grows. Each new onion
leaf emerges from the center of the neck of the onion plant. Each onion variety has a
usual number of leaves that it will produce during the season. Of most importance is to
determine not how many leaves the variety will produce but one must be able to
recognize which leaf is the “last leaf” to emerge from the neck. The “last leaf”
emergence from the neck is a signal to the grower that the onion plant is reaching the end
of the growing season.
The purpose of step one is to be able to recognize the emergence of the “last leaf” and be
able to plan accordingly for the changes that will take place as the plants mature.
Step 2: Full emergence of the Last Leaf
This step can be simply stated as the full emergence of the last leaf over the population in
the field/variety. Variation in parts of the field can occur due to soil type, water patterns,
etc.
Step 3: Soft Neck Stage
After full emergence of the “last leaf” the onion plant reaches the soft neck stage. The
soft neck stage will occur in the same percentages as percent top fall. As we know, all
fields do not have 100% top fall on the same day, so from an observation standpoint we
will be able to touch the necks and feel whether 25%, 50%, or 75% of the necks are soft.
Several important factors occur at this stage. This stage marks the end of cell division
within the onion plant; cell enlargement is the only remaining growth that will occur.
Water consumption will generally decrease by half or more! An onion field usually
spends 7 to 10 days in soft neck stage. It is important to keep good moisture levels in the
soil at the soft neck stage so we can achieve maximum bulb size, adjusting irrigation
scheduling will be necessary at this time to avoid over-irrigation. From a practical
management viewpoint it is important to bring soil moisture levels to near field capacity
at this time. If you have been behind on water all season now is a good time to “catch
up”. After the soil profile has been charged with moisture it is important to turn the water
off for several days to allow soil surface drying to avoid sour skin or other disease
problems. One or two supplemental irrigations may be all that is required after this stage
to “finish” the crop.
Step 4: Green Top Stage
This stage is the earliest proper stage where we can harvest onions into burlap for
immediate fresh consumption. Keep in mind that this stage represents 100% soft neck
and most likely 10% to 20% tops down. Some growers may want to apply sprout
inhibitor at this time. As I stated, this is the earliest stage where we could green top. It is
more common to green top at 90% to 100% top fall when we have good skin
development but plenty of green tops for sun protection.
Step 5: Tops Down
This stage represents percentage tops down. We will also make one of the most
important decisions in the life cycle of the crop during this stage – last irrigation.
The onion bulb will reach its maximum size potential during this stage and skin
development will continue to occur.
The timing of the last irrigation can be a difficult decision. The last irrigation can occur
as early as 30% tops down and as late as 100% tops down depending on the time of year,
temperatures, skin development, variety, condition of root system. From a quality
standpoint it is better to error to the dry side rather than risk quality with over-irrigation
causing sour skin, possibly black mold. Also remember the onion is still “respiring” at
this stage and pulling sugars out of the tops and moisture from the roots.
As the onions reach a high percentage top fall it is common practice for some growers to
“roll the tops” and “bust centers”. Rolling tops is an acceptable practice as long as the
onions have soft necks and can be easily tipped. Busting centers helps determine proper
lifting rod depth and ease of pulling the lifter through the field.
Step 6: Lifting
Proper timing of lifting can be a controversial subject. Just as we counted leaves to guide
us to maturity, I am suggesting that we count dry leaf sheaths to guide us to proper lifting
time. The leaf sheath is the part of the base of the onion leaf that makes the neck of the
onion. Leaf sheath material is more fibrous than the fleshy part of the leaf blade. Leaf
sheaths will mature/dry from the oldest leaf to the youngest leaf. I am suggesting, again,
that we look to the “last leaf” to guide us to proper lifting time. Logic can answer our
question of lifting time. If we lift prematurely the moisture content of the last leaf may
be too high to properly dry – it may soft rot instead of naturally drying. This soft rot
could lead to neck rot. We can examine the moisture content of the “last leaf sheath” by
stripping the oldest leaf sheaths from the neck area until we reach the center “last leaf
sheath.” Another important observation at this stage of growth is the presence or absence
of chlorophyll in the leaf sheath material and the “last leaf sheath”. Moisture content as
well as presence or absence of green material (chlorophyll) is an excellent indicator of
maturity. It takes green leaf material for photosynthesis to occur; if green material is no
longer present we must be making the transition to “respiration only”, which would by
logic represent the storage phase of our crop.
Another indicator of proper maturity is the lack of what I call “neck cavity” at the top of
the onion bulb. The top of the onion bulb should be “finished off”, that is, it should be
round and firm at the neck area and not sunken or soft. Other important factors of lifting
are root pruning and creating an environment of dry, fluffy soil and around bulbs to
facilitate drying for mechanical harvest.
Step 7: Windrowing
It is my opinion that the least amount of time spent in the windrow, the better, because of
onion quality, skin retention, sunburn, etc. If the ideal relative humidity for storing
onions is 60%, it seems logical to bring the onions into storage at greater than 60%
relative humidity. Let me explain this way. If we fill our onion storage and close the
doors and allow the system to operate for several hours and our relative humidity that is
created within storage is only 20% to 25%, it seems to me that we brought the onions in
to dry.
I am suggesting that proper windrow time may be as little as 4 days and possibly up to 10
days depending on variety and temperatures. I think that if we looked at this practice as
just an opportunity to sufficiently dry roots and to not cause moisture problems in
storage, we will bring in better quality onions into storage.
Let me suggest this scenario. Let’s assume curing time in the field to be a “constant”. In
other words, we really can’t rush the natural process of drying to mechanical harvest
stage by many days. So where do we want to spend our time? (1) Properly maturing the
“last leaf sheath” prior to lifting, (2) drying/curing in the lifted stage allowing minimum
exposure to the sun, (3) or exposing the onion to windrow time?
Step 8: Loader or topper/loader
This is the stage of onion growing that we all look forward to – harvest. Most all growers
agree on common factors at this stage. We want to minimize handling for skin retention
and mechanical damage purposes, along with bringing the onions in dry enough to finish
the curing process in storage. What is the greenest we can bring the onions into storage
for maximum quality?
Step 9: Storage Curing
To heat or not to heat? Some growers consider heating to be 90 degrees, while others
consider heating to be 70 degrees. Some growers do not heat but simply bring in outside
air that is of low relative humidity expelling the high relative humidity air out of the
storage unit. The method that is of most interest is applying 70 degree heat to the outside
air that is being pulled into the storage unit and then circulates this heated air through the
pile. This heated air that now contains considerable moisture can be expelled outside the
storage unit. By heating at a lower temperature we avoid the conditions that favor black
mold development.
One important factor that needs to be mentioned about heating storages is that if
you have the ability to heat storage for curing it is advisable to also have the ability
to cool the storage after the heat curing process. Also worthy of mention is that not all
lots and varieties benefit from heat curing, storage experience and knowing the field
history of the lot/variety are important considerations before heat curing onions.
Please remember our basic purpose for curing is to remove excess relative humidity/
moisture from the storage and the onions. Achieving proper humidity/ moisture levels
should be first priority and temperature control secondary. Curing and storing onions is
not a simple task. Get help from a qualified storage consultant-it is much cheaper than
losing your crop in the first 30 days of storage.
Step 10:
This is the stage of maturity where we no longer think of our onions as a field crop, they
now have become a storage crop and need constant attention just as they did as a field
crop.
Questions? Comments? Call Jim Christopherson, Keithly-Williams Seeds, at (509) 5476693 or (509) 531-8683.
Disclaimer: This information is presented for educational purposes only and should not
be considered as expressed or implied cultural recommendation.