December 2020 - 18

Some tips that produce high-quality onion transplants
By Thomas Ford
Penn State University
High-quality onion seed is the most critical
component in growing high-quality onion
transplants.
To grow quality transplants, growers
should select a soilless sterile potting media
for their plug trays. Most growers prefer
sowing onion seed in plug trays using 288s or
200s (if you want larger plugs). When filling
the plug trays make sure that you do not
over-fill the tray and allow enough room
for the placement of the seed. Also press
down or firm the potting media in the plug
cells prior to sowing the seed. If a grower is
sowing Spanish onion seeds, they typically
will sow one seed per cell. If the grower is
sowing yellow or red onions, they frequently
will sow three seeds per cell. Extra plants
can be removed later and used to fill in
missing cells.
Onions typically require 55 to 65 days
from sowing to transplanting in the
field under ideal growing conditions.
Immediately after sowing the onion seed,
the grower should uniformly water the
plug trays. Onion seed will germinate in
7-8 days if the air temperatures are kept
in the 65˚-75˚ F range. Cool temperatures
can lead to delayed emergence and poor or
uneven stands.
Onion transplants are subject to
stretching unless a grower is using DIF
temperature control to control growth.
Trimming onion transplants can be
Onions growing in central Pennsylvania. Photo: Tom Ford/Penn State Extension
Growers wishing to keep their onion
transplants compact should maintain their
night temperature about 10˚ F warmer
than the daytime temperature. So, if a
grower is maintaining their nighttime
temperature at 65˚ F, then they should
be keeping the temperature in their
greenhouse at 55˚ F during the day. By
using DIF, a grower can prevent their
onions from becoming spindly.
Water quality should be assessed prior
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to growing any transplants, but onions
grow best when the water source has a
pH range of 5.5 to 7.0 with a low electrical
conductivity (EC) or salt level. Moisture
management is critical in growing highquality
onion transplants. Overwatering
can lead to damping off, poor root growth,
or disease development. The edges of onion
plug trays can also dry out before the center
of the tray so a grower should monitor
moisture levels regularly. While we do not
like to see moisture stressed plants, most
successful onion transplant growers try
to confine their watering to the morning
hours. Late afternoon watering under
cloudy gray skies leads to longer drying
times which could promote foliar diseases
in the onion plugs.
Onion transplants are considered to
be light feeders. The starter fertilizer
charge in most potting media will last
2-3 weeks. About 2-3 weeks after onion
germination and after the first true leaf
has developed growers can begin to
apply nutrients via fertigation. Growers
should be applying fertilizer precisely
using a calibrated fertilizer injector.
Growers can opt to fertilize the crop
once a week at 100 ppm nitrogen or use
50 ppm nitrogen via constant liquid
feed. While many different water-soluble
fertilizers can be used, I would opt for
a 15-5-15 fertilizer. This fertilizer has
a lower phosphorus level which will
reduce the likelihood of stretching.
a controversial topic since the physical
removal of onion foliage would seemingly
encourage the risk of disease because of
the wounding process. While this is a
valid concern, growers find that trimming
onion foliage in the transplant house yields
a stronger, more compact plant that is
easier to mechanically transplant. Growers
that advocate for trimming recommend
trimming the onion plants to 4 inches in
height after the first true-leaf develops and
repeating the procedure if necessary.
Some growers have modified
lawnmowers and other equipment to trim
onion foliage. It is critical however that all
blades be kept sharp and that growers only
trim onion foliage on bright sunny days.
After onion transplants have been trimmed
all " clippings " (leaf blades) should be
removed from the plug trays.
As growers move closer to transplanting
their onions in the field, they should consider
hardening them off. Approximately one week
prior to transplanting in the field consider
moving the onions outdoors. Reduce
watering and fertilization to slow growth and
encourage hardening. Some growers like to
make a single insecticide application to the
onions outdoors while in the plug trays to
discourage onion maggots from laying their
eggs on the onion foliage.
Adapted from a presentation given by
Kevin Vander Kooi, Department of Plant
Agriculture, University of Guelph. VGN
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December 2020

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