excerpt from article in grower magazine june 2007
its about veggies but same principles can be applied to cannabis.
Plant management for generative or vegetative steering
‘Plant balance’, ‘vegetative growth’ and ‘generative development’
are the topics of this series. Previous articles gave examples of
how the environment affects the plants’ balance, and also how
environmental factors can be used to steer plants in the required
direction. These factors include temperature, humidity, light, CO2,
screening, plant management and irrigation. Earlier we dealt with
temperature effects in some detail (part 2). This article discusses
how plant management can be used to influence plant balance.
Plant balance
The term plant balance describes where plants put their energy
in. It can be described in various ways: as the balance between
leaves and fruit, or between vegetative and generative, or between ‘source’
and ‘sink’. It all has to do with energy, leaves and fruit per square
meter.
‘Source’ refers to the amount of assimilates (sugars) that are
produced in the leaves due to photosynthesis. ‘Sink’ refers to
absorption of those assimilates in growing organs, such as fruit,
plant head, young leaves and roots.
The terms ‘vegetative’ and ‘generative’ can only be used for crops
that produce flowers and/or fruit. If in a certain period, a plant
puts most of its energy into leaves, and not much into fruit, we
call this plant vegetative. At another point in time, the same plant
can be heavily loaded with fruit, while leaf growth is limited. The
plant is then called generative. Too vegetative or too generative
are not desirable states, because the plant is not in balance and the
long-term production will suffer.
A number of plant characteristics can be assessed to determine
at an early stage if a plant tends toward vegetative or generative
growth (see article 3). The best way is through crop recording,
also known as crop registration. If detected early enough, the
grower can take action before the plants get out of balance. The
results over the whole season are better if the plants stay balanced
throughout.
What influences plant balance?
Plant balance is influenced by many factors, including genetics
(variety), grafting, plant density, plant stage, season, weather,
environmental conditions, growing system, growing medium,
root-zone conditions, irrigation regime and plant management.
Some have an overwhelming effect.
Generally speaking, mild growing conditions stimulate vigorous
leaf growth, which means vegetative growth. In contrast, harsh
conditions stimulate the production of seeds (‘to help the
species survive’), which means generative development. In mild
spring weather most plants tend to be vegetative, while hot
summer conditions make them more generative. Under those
circumstances the plants have to be steered in the opposite
direction to keep them in balance. Below we discuss plant
management actions that a grower can undertake to correct
plant balance. Remember that also other actions are available,
e.g. climate control and irrigation control.
Stem density
One of the first plant management decisions is about plant
density, or better stem density. Because the overall light level is
low in winter, it is better to have a wider stem density in winter
than in summer. In summer, when radiation is high, plants are
better off in a denser planting where they shade each other a bit.
Obviously plant density can’t be changed during the year. This
dilemma can be overcome in tomatoes by maintaining an extra
stem in spring. This can be done in every third plant, resulting in
33% more stems per m2. It can also be done in steps: first one
extra stem in every fourth plant (25% more stems per m2), and
some weeks later an extra stem on the plant in the middle (giving
50% higher density than at planting).
Cucumbers are replanted a few times per year. This allows planting
wider in winter, and narrower in summer. Also the number or
shoots can be varied in cucumber. Capsicum does not have the
possibility of adjusting the stem density to the seasonal light
conditions, but there are other means to control plant balance,
e.g. number of fruit per stem.
Stem density and plant balance
Stem density has a direct effect on plant balance. This can best
be explained by ‘source’ and ‘sink’ (see above). Under high
The principles of plant balance apply to all crops, but the plant
management actions can be different.
radiation, the photosynthesis rate is very high, so the ‘source’ is
high. Keeping the plants in balance requires a very strong ‘sink’,
basically you need enough fruit to absorb all these assimilates.
This implies we need much more fruit on the plant in summer
than in winter.
This does not happen naturally in tomatoes, because there are
always three leaves for every one tomato truss, in summer and
winter. In summer, tomato plants often look very meagre, with
short and curled leaves, but with a surprisingly high production.
These are symptoms of oversupply of source and shortage of
sink. This problem cannot be cured, but it can be prevented.
To prevent it, the grower has to make sure there is enough
‘sink’ in summer. This is done by maintaining an extra lateral
in spring. Note that source and sink are considered per square
meter. An extra stem provides more fruit per m2, so more ‘sink’,
even as much as 25, 33 or 50%. Obviously the extra stem also
increases the leaf area. But given that radiation (Watt per m2)
remains the same, the source does not increase as much as the
sink. Thus an extra stem improves the ratio of sink (fruit) to
source (assimilates). It increases the production rate, and keeps
the tomato plants in much better condition. The well-balanced
plants will keep up the production after summer. Thus the total
result over the whole year will be better.
Pruning very young leaves
As mentioned above, tomato plants always make three leaves for
every truss, in summer and winter. In a very young stage, the
leaves are still ‘sinks’, because they need sugars for growth. At
that stage the young leaves and young trusses are in competition
with each other for assimilates. By removing one of the three
young leaves, the remaining leaves and the truss will get more
assimilates. Research found that the benefit is larger for the truss
than for the young leaves (see schematic drawing). This makes
the young truss grow faster, which increases the production. This
can only be done safely when the plant has enough leaf area. In
fact it is done in times when there is a surplus of source and a
shortage of sink, as described above for tomatoes in summer.
Only small leaves are removed, up to half their mature size.
Also in cucumbers some leaf pruning can be applied. For instance
large leaves on the top are removed to get more light lower in the
plants. This also affects plant balance.
Fruit thinning and truss pruning
Fruit thinning or truss pruning can be used to control the load
on the plant, and to adjust the ratio of ‘sink’ to ‘source’ in various
seasons.
Fruit thinning is an important tool in young cucumber and
capsicum plants. These plants first need to build a strong plant
body before they can produce. If planted in winter or early
spring, the light level is still relatively low, so ‘source’ is fairly
limited. Therefore some of the earliest fruit have to be removed,
to make that the sugars are invested in vegetative growth.
Truss pruning and fruit thinning are also useful tools for
plant balance control in late summer and autumn in tomatoes,
capsicum, cucumbers and other crops. The light level can be high
in late summer, but it will steadily decline in the following weeks.
Since the new fruit (or truss) has a life span of several weeks, it
will later experience poorer light conditions. A good pro-active
measure in late summer is to reduce the number of fruit (in
tomato to apply some truss pruning). This avoids the problem
Removing a young (!) leaf in a mature tomato plant makes more assimilates
available for young developing truss. Plant must have good amount of leaves.
of unbalance later in autumn, and keeps the plant in good shape
until the end of the season.
Some tomato growers apply truss pruning in order to increase
the size of individual fruit. If a larger grade is paid better than
a smaller grade, they choose to sacrifice some fruit. If this is in
summer, it is important to keep an eye on the sink-to-source
ratio. Removing fruit may increase the surplus of assimilates,
and aggravate the unbalance in the plants. The same holds for
fruit with blossom-end rot. If plants are out of balance due to
insufficient sink, it is better not to remove any fruit prematurely.
Speeding up fruit ripening and harvest green fruit
The last action to mention in this article is plant control by
‘unloading’ the plants, i.e. to shed some fruit. If there are too
many fruit on the plant for the average light conditions, the plants
will suffer. In capsicum it is ‘normal’ that plant growth comes
to a complete halt when fruit are growing out. If this lasts too
long, it can be a worry. After all, capsicum plants need to grow
further and produce new flowers to continue the production.
An effective method of stimulating growth is by off-loading
some fruit. Firstly, fruit ripening can be sped up by increasing
the temperature. Secondly, some fruit can be harvested before
they are completely ripe: harvesting green capsicums or partlygreen
tomatoes. The same principle applies to other crops too.
Unloading will make more assimilates available for the plant.
This will restore plant balance and benefit the production in the
long-term.