Late
Summer to Winter Bibb Lettuce Trial
Results from four separate trial runs of twelve Bibb
cultivars
in NFT production in Ohio
By Dr. Natalie Bumgarner
Introduction
Hydroponic lettuce production in the United States now
encompasses a wide spectrum of lettuce types and cultivars. Although there is
an increasing amount of diversity in the cultivars being produced in hydroponic
greenhouses, Bibb cultivars still occupy a large percentage of the market. They
are also often the first crop produced by many beginning growers around the
country. While some growers tailor their
cultivar selection to seasonal conditions, many growers at a variety of scales
produce a single cultivar for the whole year that is adapted to a range of
conditions. Both of these production patterns, though, require the growers to
be familiar with the growth habits, characteristics, and productivity of the
cultivars. This trial was designed to evaluate a selection of Bibb lettuce
cultivars through a range of late fall and winter conditions to evaluate their
potential for greenhouse growers in the Midwest and northeast. Cultivars were
obtained from a variety of seed suppliers to represent a broad selection of
cultivars available to lettuce producers.
Methods and Management
Primed and pelleted seeds were seeded by hand in
pre-moistened 1” x 1” x 1 ½” rockwool cubes. Seeds were germinated in clear
water in seeding trays in the nursery. Nutrient solution was added in the
nursery 7 days after seeding. Seedlings were produced in flowing nutrient
solution in the nursery for an additional week to two weeks before
transplanting. After transplanting, lettuce plants were grown out in the
channel for four to five weeks prior to harvest. The first two runs were
carried out on the typical summer schedule (2+4 weeks) while the third and
fourth were carried out in a more winter schedule of (3+5 weeks). The nutrient
solution was continually cycled through the CropKing Fertroller where automatic
pH and EC adjustments met programmed solution set points. The pH was maintained
at 5.8 by the addition of dilute sulfuric acid. EC was maintained at 1.8 by the
addition of concentrated fertilizer solution and source water.
* It is important to note that these trials were carried out
over set periods of time and harvests were made regardless of plant readiness
for sale. Clear comparisons were the main goal, and this resulted in low
weights for some cultivars. Also, no supplemental lighting was used after
transplanting even under these low light northern conditions. In a commercial
operation, more time would have been give these heads to reach a saleable size
or lighting would have been added to increase growth rate and crop
quality.
Concluding Thoughts
As I look over these interesting numbers from four
consecutive runs of this Bibb lettuce trial, there are a couple thoughts that
jump to the top of my ‘important take-home points’ list.
The first and likely most glaring message is that light, not
surprisingly, is pretty important for lettuce growth and quality. As can be
seen by the average solar radiation and weights in the previous tables, low
winter light in northern climates can be a significant hurdle. It is clear that
lengthening production cycles by two weeks in the winter didn’t produce similar
sized heads to the late summer/early fall run. The fourth run had just a little
more than 33% of the average solar radiation present in the first run (In
future blog posts, we will break these numbers down to daily integrals).
Average temperatures in the first two runs were a little higher as well (due
partly to solar gain). However, the individual contributions of light and
temperature are difficult to clearly separate.
The pictures that follow drive home the point that
light/temperature relationships are crucial for lettuce growth and development.
Supplemental lighting would have changed this equation substantially, but that
is another broad topic altogether. Also,
additional time in the channels would have made many of the smaller Bibb heads
quite marketable. Production timing and seasonal factors should not be overlooked
if your goal is to produce consistently sized heads throughout the year in
northern climates.
The second important theme that is apparent in these yield
numbers and the pictures that follow is the need to understand the variability
in cultivar performance. While all the cultivars trialed here were bibb or
butterhead lettuce, not all were bred for hydroponic or greenhouse production.
In controlled environment production, consistency is key. Consistency is
important not only within a harvest period, but also through the seasons. The
second yield table shown above illustrates this point. The deviations across
the 4 runs as well as the % weight loss from the highest yielding run (1) and
the lowest yielding run (4). It is clear both in these variable yields and in
the photos below that in our location without supplemental lighting, some of
these cultivars are not suitable for year-round production. The larger framed
lettuces tended to be the most variable across runs and were visually the least
appealing under winter light conditions while many of the bibbs bred for
greenhouse production were more consistent in weight and visual quality across
the trials. These trends held true in both the green and the red cultivars.
Knowing your cultivars is crucial in creating an appealing
and consistent product throughout the year!
