Building the Chassis

April 19, 2006 - 08:22

Final crop quality often depends on
the chassis built within the canopy.
The chassis refers to the architecture,
or scaffold, that develops as a result
of pinching. The scaffold is influenced
by several factors including light interception,
pinch technique and pinch timing. In this
article, I will review the basic principles of branching
and demonstrate how you can manipulate
spring crops with pinch timing and technique.

Supply And Demand

A basic principle of branching is that the
number of developing shoots is proportional
to the amount of resources available
to support those shoots. In other words, if
the plant has a number of resources available,
most axilliary buds below the pinch
will develop into lateral shoots. In contrast,
if resources are limited, fewer shoots
will develop. The most important resource
is light intercepted by the leaves; however,
root growth and the supply of water and
nutrients are also important.

Light

Light interception depends on the
amount of light delivered to
the canopy and the spacing of
individual plants. When plants
are grown under low light levels
the number of branches
decreases. Figure 1, above,
shows the increase in branching
of vinca bedding plants as
the light level increases.

Spacing impacts branching
by altering both light interception
and the light quality delivered
to axillary buds. Figure 2,
right, demonstrates how spacing
and plant size impact light
interception. The actual data
were collected on New Guinea
impatiens (3- and 6-inches tall)
grown in 4-inch pots and
placed pot-tight (high density) or at staggered
(low density) spacing. Light interception was calculated
by making light intensity measurements
above and below the canopy. Light interception
obviously increases as plants get larger or spacing
is tighter; however, it is very interesting to
observe how the light delivered to individual
plants increases as spacing increases.

While this may be intuitive, it is noteworthy to
see that small plants placed at wider spacing
decrease the total light interception from 87 to 56
percent, but the interception per plant increases 30
percent (0.087 to 0.113 moles per day per plant).
Similarly, large plants placed at wider spacing
decrease light interception from 97 to 70 percent,
while the interception per plant increases 45 percent
(0.097 to 0.140 moles per day per plant).

Additionally, a tight canopy will reduce the
amount of red light that filters down to the
axillary buds. Buds that perceive filtered light
(red less than far red light) do not break as well
as buds that intercept direct sunlight (red light
equals far red light). Thus, branching is inhibited
by tight canopies.

Spacing is a critical economical issue, so we
are not suggesting that plants be grown at luxurious
spacing. Rather, we are trying merely
to demonstrate that crowded plants have
considerably lower light interception per
plant and a low red-to-far-red light ratio;
thus, it is to be expected that branching
will also be greatly impacted. The takehome
message is that the spacing at the
time of pinching will impact the plant scaffold.
Overly crowded plants at the time of
pinch will certainly have lower shoot
counts, so spacing needs to be considered
in regard to the time of pinching.

Container Volume

The impact of container volume
on branching is not often
considered; however, plants
grown in small containers may
not branch as well as the same
plants grown in larger containers.
Figure 3, page 48, shows
African marigolds grown in five
different containers. All plants
were grown at 6x6-inch spacing,
and no pinch was performed.
Container volume affects the
water and nutrition available
for plant growth. The gaseous
environment in the growing
media is also likely to be different.
Interestingly, the changes in
growth occur before any significant
root restriction occurs, so
being rootbound is not likely to be the causal factor.
It may not be clear which factor is the most
important, but it is clear that container size
impacts branching, thus plants pinched prior to
transplant may not branch as well as those
pinched after transplant due to the container volume
at the time of the pinch.

Pinch Height

While pinching appears to be a
relatively simple concept, it really
has a big impact on the final plant
quality in terms of branch number,
plant height, flower number and
timing of flowering. To demonstrate
this, we examined the effect
of pinch height, pinch hardness
and pinch timing on nemesia.

Pinch height refers to the node
number remaining on the primary
stem after the first pinch. The
number of nodes left on the stem
below the pinch obviously
impacts shoot number, but it is
less obvious that this also affects
plant vigor and flowering. To
demonstrate this, we transplanted
several species and then pinched
them low, medium and high.
These are relative terms. If a species produces
many nodes prior to setting a terminal flower,
such as osteospermum, then a low pinch left
three nodes, a medium pinch left six nodes and a
high pinch left nine nodes below the pinch. If a
species produces a lower node number prior to
flowering, such as nemesia, then a low pinch left
one node, a medium pinch left three nodes and a
high pinch left five nodes below the pinch. The
same amount of tissue was removed during each
of the pinches. The pinches occurred on different
days, since it took longer for more nodes to
develop prior to pinching.

Figure 4A, right, demonstrates the results
observed on nemesia. The low pinch (to one
remaining node) produced two shoots per plant,
while the high pinch (to five remaining nodes)
produced 10 shoots per plant. The node closest
to the soil surface was always the most vegetative
and the slowest to flower. Vegetativeness
was quantified by recording the number of
nodes that formed on the shoot below the terminal
flower, so the bottom node typically formed
shoots that had seven or more nodes. The highest
node following the pinch was always the
most reproductive, so these nodes produced
shoots with 4-6 nodes.

In general, pinch height has little impact on
time to flower but has a great impact on the
number of flowering stems per plant. For example,
when two nemesia cuttings were grown in
a 4-inch pot, the plants that were pinched to
one, two, three, four or five nodes
had 3.7, 3.9, 5.9, 6.7 and 11.3 flowering
shoots, respectively, per pot
after 35 days.

Pinch Hardness

Pinch hardness refers to the
number of nodes removed during
the pinch, while the node number
remaining on the stem after
pinch remains the same (Figure
4B, right). Thus, a soft pinch
resulted in one node being
removed from the shoot, while a
hard pinch resulted in three
nodes being removed from the
shoot. Hard pinches are done on
later dates than soft pinches since
additional nodes must develop
on the stem prior to the pinch.

The time from pinch to first
flower was 18 days regardless of
the pinch hardness; however, the hard pinch
had a later pinch date that resulted in extra
production time. For example, the average
shoot for the soft pinch flowered in 36 days,
while the hard pinch averaged 44 days to
flower. The shoots that develop on the hard
pinched plants also had an additional 1-2
nodes per stem. Shoot number per plant was
not affected by pinch hardness.

In general, pinch hardness does not affect total
shoot number, but does have a significant impact
on the timing of flowering. For example, when
two nemesia cuttings were grown in a 4-inch pot,
the plants pinched to three nodes that had one,
two or three nodes removed had 8.9, 8.5 or 4.8
flowering shoots per pot after 35 days (all three
pinch treatments had 9-10 total shoots per pot).
Soft pinches minimize time to flower, while hard
pinches increase time to flower. This occurs
because the uppermost node below a soft pinch
produces a shoot that has a lower node count
prior to forming a terminal flower.

Pinch Hardness And Pinch Height

In the preceding two examples, the different
pinch treatments had to be performed on different
dates. This third example demonstrates the interactive
effects of pinch height and hardness on
pinches performed on the same date. For example,
all plants had six nodes at the time of pinch and
had pinches that were hard and low to one node
or soft and high to five nodes (Figure 4C, above).

The plants pinched to
one node averaged 47 days
to flower, while the other
four treatments averaged 36
days to flower. The bottom
node was always the slowest
to flower, and when the
plants were pinched to just
this one node, flowering
was quite slow. The bottom
node produced shoots that
averaged 8-9.5 nodes per
stem prior to flowering,
while the nodes closest to
the pinch formed shoots
that averaged 4.2-6 nodes
prior to flowering.

Pinch hardness and height
also had a significant impact
on the number of flowering
shoots per plant. For example,
when two nemesia cuttings
were grown in a 4-inch
pot, the plants pinched to
one, two, three, four or five
nodes had 0, 3.7, 3.2, 7.2 or
11.3 flowering shoots per pot
after 37 days.

In summary, branching
within the scaffold will be
improved if the light levels
are relatively high and the
canopy is open enough to
allow light to penetrate into
the axillary buds. Pinch
height should be based on
the number of shoots needed
to produce a satisfactorily
full plant, and then a soft
pinch should be performed.
This maximizes shoot production
and minimizes time
to flower. Pinching too low
results in too few shoots
(and may require a second
pinch) and those shoots may be more vegetative
than desired. Pinching too high can delay flowering
by wasting time developing nodes on the primary
stem rather than developing lateral shoots.

Typically, the worst-case scenario is to grow
a plant with two low pinches. By this we mean
to pinch the primary stem to a low node number
and then follow up with another low pinch
on the secondary stems. The resulting plant
will have a relatively slow growth rate due to
the small leaf area remaining after the pinch.
Also, the shoot number will be low, and those
shoots will be more vegetative, so flowering
will be delayed.

About The Author

Jim Faust is associate professor at Clemson
University, Clemson, S.C. He can be reached by Email
at jfaust@clemson.edu.

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