Pass the Pruners: How to Control Plant Growth
By Lynette Morgan 13 March 2019
Takeaway: While it can be a little unnerving to make that first cut to an otherwise large, healthy plant, controlling plant growth is essential for many commonly grown hydroponic crops. Fortunately, pruning and training plants so that they grow where you want them to grow is relatively easy. All it takes is some basic knowledge and a little bit of practice.
Manipulation of plant form and function is as old as agriculture itself. Ever since plants were first cultivated for food or ornamental value, growers have been snipping, shaping, trimming and tying to achieve the most from their crops.
These processes are even more vital in the confines of an indoor garden where rampant growth rates can turn into a jungle in no time, which leads to reduced productivity and increased disease occurrence.
Why Should I Control Plant Growth?
The reasons for pruning and training vary between different crops. For many crops it is simply to create a compact, productive plant with the optimal ratio of flowers/fruits to leaves and avoid overcrowding of stems and foliage.
For others it may be more regimented or for purely ornamental reasons, such as the extreme pruning and training required to grow gnarled and ancient-looking bonsai trees. Other reasons may also be aesthetic—removal of old, yellowing foliage is part of the basic maintenance program of many plants.
Assisting environmental control is another aspect of pruning and training that is particularly important with an indoor garden where humidity control and airflow around the base of plants can become an issue.
Removal of lower leaves, stem thinning and height reduction under artificial lighting all assist with maintaining more ideal growing conditions for plants.
Vine crops such as tomatoes and cucumbers produce many side shoots, each of which will set fruit. If all shoots (laterals) are left on the plant, the result is a dense, tangled canopy with a large number of undersized fruits.
When the number of fruiting shoots is reduced with pruning, the remainder can be supported with training methods so that large, high-quality fruit are produced.
Apical Dominance Removal
Removal of apical dominance sometimes referred to as pinching out of the main growing point or tip is typically carried out to stimulate the development of side shoots and create a more compact and branched plant form.
Examples of crops where tip removal is recommended are many hydroponic herbs such as sage, basil, thyme and rosemary, and flowering plants such as chrysanthemums and many potted houseplants, where a shorter, more compact plant with more foliage or flowers are desirable.
The most extreme example of pinching out is with crops such as tea (Camellia sinensis) where the growing tips are continually harvested for tea production, resulting in a proliferation of a large number of young shoots for harvest.
Hydroponic plants that are successively harvested for their foliage while continuing to grow are being pruned with each cut, forcing the development of more leaves until the plant becomes exhausted and needs replacement.
Pruning of Leaves, Flowers & Fruitlets
Leaf pruning is common in many hydroponic plants and may be used to adjust the vegetative (leaves and shoots) to generative balance (buds, flowers and fruits) of the plant.
Leaves may also be removed to maximize airflow and prevent disease or expose more fruits to sunlight, which accelerates the rate of ripening. Side shoots (known as laterals), are often pruned to create dominance of the main shoot of a plant, which results in a taller, less branched growth habit.
Side shoot removal also limits the number of flowers and buds carried on many plants such as tomatoes, cucumbers and capsicums, which helps improve the size and quality of the fruit permitted to develop.
Buds and flowers are often pruned on plants, either to stimulate vegetative growth or regenerate more foliage after a period of heavy fruiting.
An example of this is the removal of the first flower buds on young, small strawberry or pepper plants to allow sufficient canopy size to develop before the plant can support fruit growth. Disbudding or removal of flowering shoots is carried out on some types of cut flowers, which maximizes bloom size and quality.
Gardeners also carry out this process, particularly where exhibition-sized flowers are being produced in species such as chrysanthemums, dahlias, carnations and roses. Removal of all but one of the flowering buds on an individual plant or stem results in a significantly larger size of the remaining flower as the assimilate supply is only directed into one bud.
Deadheading is another form of flower removal, largely carried out on flowering plants and ornamentals rather than fruiting crops. It stimulates more vegetative and flowering growth on the plant. Some hydroponic crops such as basil benefit from the early removal of flower buds so that the plant focuses energy on leaf growth only.
Fruit pruning or fruit thinning involves the removal of a selected number of fruitlets from a plant so that the rest can reach an acceptable size.
An example of this is cluster pruning with hydroponic tomatoes, which restricts the number of fruit per truss to about four to six tomatoes, often from as many as eight to 12 fruitlets. Grape bunches may also be cluster pruned to obtain the ideal grape size and improve the flavor and dry matter content (sugars) of the fruit.
This increases their sizes considerably and is particularly important for markets requiring premium-grade fruit.
Root Pruning and Training
Manipulation of the root system is a less commonly used form of growth control and involves root zone restriction or root removal and trimming. Root pruning helps slow overly vigorous growth and keeps plants more compact.
It is more commonly used in the nursery industry to control tree size and stimulate a more compact, stress-resistant plant.
The aim of root pruning is to cut both fine and large roots to reduce the absorption of water and nutrients, which limits shoot growth and alters the hormone balance of the plants. Root restriction is a training method often used in hydroponic production, particularly of larger, longer-term plants.
By controlling the root zone volume, vegetative vigor is restricted, allowing the plant to partition more assimilates into flower and fruit production. Use of air pruning or root trainer pots creates a denser fibrous root system that is thicker and contains more carbohydrate reserves to assist plant survival once it is transferred to a new home, reducing transplant shock.
Root restriction containers or grow bags are particularly useful for growing small fruiting trees indoors because they assist in keeping trees compact and highly productive.
Plant Pruning Equipment
Pruning equipment for a small indoor garden can be as basic as hand-pinching small, soft growing points. However, pruning knives and shears are recommended for clean cuts of larger stems, particularly of peppers and tomatoes where tissue tears increase the risk of infection by Botrytis and other pathogens.
Pruning knives or shears should be regularly disinfected with bleach to prevent the spread of a number of diseases from plant to plant, including viruses that are carried in plant sap.
Plant Training and Support
While pruning may change the shape and vegetative to generative balance of a plant, training methods are largely used for plant support, improving light interception and airflow around the canopy, maximizing the use of vertical space and keeping fruits above floor level.
There are a number of different training methods used for various hydroponic plants:
- Simple stakes and wooden trellises
- Strings attached to an overhead wire system
- Layers of horizontal plastic netting for flower crops
- Fruit slings or supports for large melon fruits
- Custom-made spool training systems for crops like tomatoes
Tomatoes – Hydroponic tomatoes are usually grown as long-season crops of indeterminate plants trained to an overhead wire with the use of support strings wound around or attached to the main plant stem.
Indeterminate tomatoes continue to grow upwards from the main stem until the growing tip is removed when it’s time to finish the crop. To allow for this, a training system of layering can be used.
Layering involves permitting the main plant stem to continue to grow upwards, producing successive trusses of flowers and fruits while the harvested portion of the lower stem—with the leaves removed—is laid down along the floor or tied to low supports to keep it out of the way.
Only the upper 6 ft. of a productive stem is held upright and supported for further growth. This training system allows the plant to produce a large number of successive fruit trusses without requiring crop removal and replanting once the height of the top training wire has been reached.
Cucumbers and Melons – Hydroponic vine crops such as cucumbers and melons can be trained similarly to tomatoes, grown for many months before the crop is stopped by removal of the main growing point.
Hydroponic cucumbers can be trained in a number of ways, the traditional method being the umbrella system where all lateral branches are removed form the main stem as they develop until the plant reaches the overhead support wire.
The growing point is then removed and two strong side branches are allowed to develop. They are trained back downwards, setting fruit along their length.
Another cucumber and melon training system is the canopy method where plants are trained up and over a series of wires to create an overhead canopy from which the fruit hang and develop. Vine crops such as cucumbers in an indoor garden may be trained to suit the size and dimensions of the area and shoot growth terminated as required.
Peppers – Hydroponic pepper plants are trained similarly to tomatoes, with more main shoots (leaders) allowed to develop to carry fruit. Depending on the cultivar and density, pepper plants are trained to maintain two to four main leader shoots, each of which is supported with strings attached to an overhead wire.
This helps prevent stem breakage that is common in pepper plants once large fruit are close to maturity. Pepper side shoots have the growing points removed after the production of three to four leaves so all the fruit is produced on the two to four main stems.
Flowers – Hydroponic flower crops, which require long, strong stems to support large blooms, are usually supported with flower netting or plastic trellis stretched horizontally across the crop in several heights through which the stems grow as they lengthen.
Hydroponic cut flowers such as carnations and chrysanthemums are usually grown with flower netting supports that promote long, straight stem development and prevent the listing of top-heavy plants.
Understanding how to direct, control and support plant growth is an important skill for indoor gardeners to develop, particularly since space is often highly restricted and dense canopies are prone to getting out of hand.
While much of plant pruning may involve removal of older leaves to assist airflow and unwanted side shoots, the ratio of foliage to flowers and fruits can be manipulated to maintain the correct plant balance, giving maximum productivity, healthier plants and higher-quality produce.