Dr. Lynette Morgan offers her tips for balancing vegetative and generative growth through crop steering.
Growth balance is something many indoor growers may not be tuned into—after all, plants are programmed to grow leaves, flowers and fruit with little input from us. However, some of the tools used by the commercial greenhouse producers of fruiting crops can be applied on a smaller scale, and crop steering is a useful technique to know and understand. Crop steering works on the principle that there are two different types of growth with flowering and fruiting plants: the vegetative phase, which refers to foliage and stem growth, and the generative phase, where flowers form and fruit begin to set and develop. Balanced growth means the plant produces just enough foliage to support a good number of developing flowers and fruit, thus maximizing yield without putting a lot of extra energy into unnecessary amounts of foliage.
What is vegetative growth?
We often hear about plants that might have gone overly vegetative—a common occurrence under lush hydroponic growing conditions with plenty of water and nutrients. Overly vegetative crops are fairly easy to recognize. With plants such as tomatoes, the stems are often very thick—sometimes described as being like tree trunks—and the foliage is lush and light green with large leaves. Overly vegetative growth means the plant doesn't have a lot of developing flowers or fruit, and is instead sending most of the assimilate produced via photosynthesis into further leaf growth. Any flowers that develop may be small and weak and may even abscise, as leaf growth is favored over generative growth in this condition. The canopy of an overly vegetative crop often appears very dense, with many long leaves, and will present fast growth and rapidly increasing plant height, but few mature fruit and sparse flowering.
"Flowering and fruiting crops large and small will give optimal yields if the vegetative/generative balance is correct."
Strong vegetative growth is common in young plants, which naturally favor a high degree of foliage development. However, certain other conditions also favor vegetative growth, and this sort of development is common with plants growing under mild conditions where there is little or no plant stress. Using substrates that retain a lot of moisture—such as fine grade coconut fiber—and running lower-nutrient EC levels also encourages vegetative growth, as do reduced light levels, high humidity and plenty of water from small frequent irrigations, especially with young plants or those grafted on vigorous root stocks.
What is generative growth?
A plant that has become overly generative is one where the assimilate being produced is largely being directed to the developing flowers and fruit at the expense of further leaf development. An overly generative plant may also have yields restricted due to a lack of foliage development, which is required to provide the assimilate required for flower and fruit development. In tomatoes, this is often described as thin growth, with leaves being small and sparse and petering out at the tops. The plant stem becomes thinner, as opposed to the tree trunk-like stem diameter of plants, which are overly vegetative, and sometimes the flower clusters are short and bent in a downwards direction. Usually generative plants have a high number of flowers or fruit, which attract the bulk of the assimilates produced by the leaves. While a highly generative plant may seem like a good idea in order to maximize yields, in reality this sort of lack of balance means there is insufficient foliage to support the number of flowers and fruit which form, and in many crops this will mean smaller fruit size and reduced yields.
Factors that favor generative development include older plants with heavy crop loading, growing in warm conditions with high light levels, CO2 enrichment and low humidity or VPD. Freely draining growing media and restricted moisture, higher EC and harsh growing conditions caused by light and heat also tend to encourage generative development.
Getting the balance right
Experienced growers will soon learn to tell if a crop is looking a little too vegetative—often before flowering begins—and can implement some measures to keep things on track. With tomato crops there are some basic measurements that can be used to determine the vegetative-versus-generative balance. A tomato plant with a balanced growth habit tends to have a stem thickness of approximately one third of an inch in diameter at six inches below the growing point. A much larger diameter indicates the plant is heading in a vegetative direction, and a much thinner stem means the plant is going generative. A number of plants should be measured and the average reading taken, as there is always some natural variation between plants in a crop.
Steering the plants in the right direction
Fortunately, there are many tools and techniques that can be used with a protected-environment hydroponic crop where the grower has control over all growth factors. Tomatoes and capsicum have the best selection of cultivars that have been bred for vegetative or generative or even balanced growth habits, and it's worth selecting these for certain growing conditions. In indoor gardens many tomato and capsicum plants respond to the highly protected environment by going more vegetative, so selecting cultivars with a generative growth habit often helps in obtaining better yields. Tomatoes grown in a summer glasshouse with high levels of natural light and a heat stress are better selected from the more vegetative cultivars in order to give a better balance of growth under the harsher growing conditions. Cultivar alone, however, is usually not enough to guarantee a good growth balance, and some degree of growth manipulation is often required for many indoor plants.
Steering with temperatures, light, humidity and CO2
As a general rule, lower light or shading favors vegetative growth and high light intensity favors generative growth. High humidity favors vegetative growth, low humidity favors generative growth. CO2 enrichment favors generative growth once flowering or fruit set has occurred as it allows greater production of assimilate from the same leaf area. Most growers understand these basic concepts and their effects on growth balance, although a more advanced tool is available in the use of day/night temperature differentials (DIFs), which can be used to steer a crop back into more balanced growth. DIFs work well in indoor gardens where the grower has good control over temperatures, and they are also used to promote the transportation of assimilates from the leaves into the fruit. High day temperatures tend to promote stretching and elongation of the plant, as well as an increase in leaf area. Night temperatures do not play a role in this particular relationship, but the difference between day and night temperatures is important to flowering and fruit development. Environmental conditions also play a role—low light levels and warm temperatures promote stem elongation and a tall weak plant, so the temperature should be kept cool and matched to the light levels. A low DIF (where night and day temperatures are similar) is used to stimulate vegetative growth, while a greater DIF (where night temperature is much lower than the day temperature) results in more generative growth. This means it's often a good idea to impose day and night temperatures, which are fairly close to each other early in the plant's life to stimulate good foliage growth, then switch to a greater DIF with much lower night temperatures as the plant comes close to flowering and fruit set. From then onwards, tomato and capsicum growers may change the DIF settings to push either vegetative or generative growth as required.
Steering with EC and moisture levels
Along with temperature DIFs, conditions in the root zone act to steer plants towards more vegetative or generative growth to help maintain balance. Growing substrates with a high water-holding capacity—such as fine-grade coconut fiber, peat and vermiculite—tend to favor vegetative growth, while those with a much freer draining nature, such as coarse perlite, encourage more generative growth, although irrigation management also plays a large role in root-zone moisture levels.
A high moisture content in the growing media created with the use of frequent irrigation and a relatively low EC of the nutrient solution will promote water uptake by the plant and encourage vegetative growth. Applying controlled plant stress with the use of higher EC or deficit irrigation practices will have a more generative effect. Deficit irrigation may include reducing the volume applied at each irrigation, allowing more time between irrigations and allowing the media to dry slightly overnight by restricting early morning and evening irrigations. This type of deficit irrigation and the use of high EC to force plants into generative growth must be used with caution, however, as moisture fluctuations in the root zone can lead to an increase in fruit splitting and cracking, and are also associated with an increase in blossom end rot development in tomatoes and peppers under certain growing conditions.
Sizing up tomato fruit
The warm parts of tomato plants (growing points, fruit, leaves) attract more assimilate than colder parts do. A small difference in temperature can make a considerable difference to the distribution of assimilate within the plant—thus warm fruits attract more sugars for growth than those that are cooler. A good technique used by growers to boost fruit growth and size is the pre-night drop. This process involves allowing the plants to receive increased warmth in the late afternoon. This heat is then absorbed by the fruit tissue, while sugars accumulate in the photosynthesizing leaf tissue. During the evening the temperature is then dropped back quickly by several degrees. The drop in temperature causes the thinner mass of the leaves at the top of the plant to cool quickly, while the fruit, having a much larger mass, remains warm for longer. The cooled leaves then unload their assimilate sugars to the warmer fruit tissue, forcing more sugars for growth out of the foliage and into the fruit. Overall, the 24 hour temperature average is not affected by this process, so it does not affect plant internode length or stem diameter. The rate of assimilate importation into the developing fruit determines fruit size and the amount of assimilate that is sent into the fruit is highly dependent on the temperature of the fruit tissue—the warmer the fruit tissue, the more sugars are imported into the fruit. The optimal fruit temperature for sugar importation is between 73o and 77oF. As an example, the pre-night temperature drop might be from a late -afternoon warming of up to a maximum of 77oF, followed by a drop of three to four hours at 60o to 62oF in the evening. From then onwards normal night temperatures are run. This technique works best in indoor gardens that have good temperature control, and is highly effective provided it is carried out correctly—some growers use infrared thermometers and internal fruit sensors to check leaf and fruit temperatures when they first attempt this process, to ensure that there is a sufficient temperature difference between the leaves in the head and the warmer fruit below.
Plant steering with peppers and crop unloading
Peppers tend to grow in flushes, with periods of leaf growth alternating with periods of fruit growth. This usually results in fruit being harvested in flushes during the first few months of the crop, which is a normal growth cycle, but not one that is favored by growers wanting a regular supply of fruit. Usually, after the second flush of fruit is harvested, pepper plants will then resume a more balanced state of growth, with a consistent pattern of vegetative and generative growth. There is little that can be done about this natural cycle of vegetative and fruit growth in peppers, but if the halt to vegetative growth goes on for too long, yields can be reduced and future harvests delayed. One effective method of forcing both pepper and tomato plants into unloading some fruit so that vegetative growth can be speeded up is to increase the temperature. Another method is to harvest some fruit before they are completely ripe, which may mean harvesting fruit from young pepper plants when mature green rather than waiting
for the first signs of coloration.
Summary
To direct plants into a more vegetative growth habit:
Shade crop or reduce light (but not so low as to severely restrict photosynthesis); apply frequent irrigations; allow less dry-down of the root zone overnight; use moisture-retentive fine-grade growing media; lower EC; increase humidity; reduce day temperature; use a low DIF (similar day/night temperatures).
To direct plants into a more generative growth habit:
Apply a little stress—increase EC, restrict moisture in the root zone, allow media to dry slightly more overnight. Increase light levels and temperature, increase CO2, drop back humidity. Use a freer-draining media or more careful control over irrigation, drop back irrigation frequency and amount. Use a greater DIF (night temperatures much lower than day temperatures). For sizing up fruit, use pre-night temperature drop method to force assimilate out of leaves and into developing fruits.
