By Dr Lynette Morgan

Introduction

Growing plants, whether it’s for food production or ornamental purposes relies on the grower providing sufficient light, nutrients, water, and a good growing environment. However, many plants if left to their own devices often don’t provide the shape, form or most attractive display that could be possible with a little intervention. Typically flowering plants grown in an indoor system need some assistance to give them that dense, luxuriant, bushy and colorful compact garden that looks truly amazing. Many fruiting plants also require some pruning and training to maximize fruit size and quality as well as keeping the plant size under control. 

Plant forms and development

Most of the small flowering plants grown with the use of indoor hydroponic systems have the ability to produce numerous flower buds. What can occur however, is that a young cutting or seedling, particularly one that has been slightly `stressed’ during the propagation stage, may simply produce a few leaves, then a flower bud while still very small in size. This effect of `pre-mature’ flowering on an overly dwarf plant is common where certain plant species have been grown from clones or cuttings. The type of cutting material taken has a large effect on this occurrence. If cuttings are taken from mature plants that our in flowering or about to flower, they have tiny flower buds already formed inside the growing point, even though they may not be visible. Once the cutting has been taken, plant is triggered back into growth, and it is the flower bud, which is present in the growing point that starts to burst, rather than any vegetative buds. This means that while the young cutting or plant has formed a root system, it does not produce many further leaves, but puts all its limited energy into the production of that flower bud in the tip. The result is a stunted, dwarf plant with a large flower, which uses all the reserves the plant, can produce and this in turn restricts the development of any further leaves or flowers.

A similar occurrence happens with seed raised flowering plants which may have suffered some form of stress during propagation - such as cooler or warmer than optimal propagation conditions, high ppm (EC), moisture deficient, salt stress, pathogen attack (pythium and others) too much light etc. These stress forces the plant into pre mature flowering while still very small and immature - which in turn retards the development of further vegetative growth. The result is not the dense bushy green garden of flowers the grower was trying to achieve.

The opposite problem can also occur in indoor flowering gardens - the plant may in fact shoot upwards rapidly before and during flowering, resulting in too much vegetation and a plant that is too large for the system. This is common where the plant has a single large (or just a small number) of individual shoots - all the assimilate produced by the plant goes into the growth of just a few shoots which then become very tall and large before the flowers have a chance to form. This type of growth is common where the plant has a very dominant central shoot that tends to restrict the development of side branches and slow the production of flowers.

Ideally, small flowering plants in an indoor garden should be compact - branched, self supporting, uniform in size and shape and flower predictably within a few weeks time. However, leaving the plants as they are and hoping they will all just naturally grow uniformly in size and shape at the same time is a little unrealistic in an indoor garden. It is far better to prune and shape the plants to the requirements of the garden and be guaranteed of a good display.

Plant pruning and manipulation

Good plant manipulation is based on understanding how a plant will react to pruning.

When taking cuttings for clones, always try to select vegetative only shoots, this can be difficult, as flower buds can be too small to be seen, but never select a cutting that has an obvious flower bud in it (or remove this flower bud before rooting the cutting). Ideally cuttings taken from a mature plant should be selected from young, recently matured shoots which are not overly soft and succulent, avoid those at the top of the plant and take older side shoots if possible. Choosing only shoots, which have not yet developed flowers or flower buds, will mean the cutting has more reserves available for the development of new roots and a stronger new plant. Any flower buds that do become visible after taking a cutting or during the process of rooting, or even soon after rooting must be removed until the plant is large enough to support flowering.

Cuttings or young plants which seem to be forming a flower bud or flower in the growing point (flower buds are typically fatter and rounder than vegetative buds) need to have this flower bud pinched out, otherwise a stunted dwarf plant will result. Pruning out the flower bud will force other dormant buds on the plant stem to burst into life. These stems will later also flower, but flower buds should be pinched out, until the plant has reached the correct size for flowering (small plant harvest 30 cm (12") - 40 cm (16") from the root zone).

Some plant species will only flower if exposed to short day hours. If a short day plant which needs less than 12 hours light per day is grown in long day lengths, it to will not flower. For this reason it’s always a good ideal to check out whether the plants being grown in an indoor garden have any day length requirements for flowering and adjust the timing of any artificial lighting to promote this.

For young plants which seem to be producing overly tall, strong shoots before any flower buds are seen, prune these back by removing the growing point before it gets too tall. The 15 - 20 cm tall stage is a good time to prune out the growing point of these dominant shoots. The plant will then branch out, producing a bushier, more compact shape, and ultimately more flowers on the side shoots for a better display in the garden. Any shoots which appear to be getting too tall or large for the garden should always be pruned back to keep the compact shape of the garden, and this is one of the best ways to promote new flower bud formation.

The final type of pruning that should regularly be carried out on a garden of small flowering plants is `deadheading’ or the removal of old flowers and spent foliage. Deadheading serves a number of purposes. Firstly, it maintains the appearance of the garden by taking out spent and fading flowers and old leaves. Secondly, it removes any decaying tissue that might otherwise be a site for disease to take hold and infect the plant. Thirdly it makes more plant assimilate available for new flower buds and leaves, by removing any developing seed. And finally it helps trigger the development of new flower buds, shoots and foliage which keeps the plants in the display looking fresh and healthy over a greater length of time. Deadheading and shoot pruning can be carried out at the same time in order to keep the garden in the same compact shape of the correct height with uniform plant growth.

Fruiting plants

Many of the fruiting plants grown in hydroponic systems also need `growth control’; both to keep vegetative growth in check and to push assimilates into the fruit, rather than excessive foliage growth. The tomato plant is one such example. If left to its own devices, most indeterminate tomato types will soon produce a large number of `laterals’ or side shoots creating a huge tangle of foliage with few weak flowers and fruit. Ideally, these types of plants need to be pruned on a regular basis and this involves removing any laterals that form, while training the main stem upwards as it produces more fruit trusses. Lateral shoots are small `stems’ that form in the axils of the large leaves on the main stem. If left on the plant, these lateral shoots rapidly grow outwards - they are often thin, weak and drain reserves from the main plant stem and trusses. They also create a large, weak, bushy type of plant, which is difficult to train and support. Removing these laterals creates an easy to support, strong main stemmed plant with good fruit trusses or well-sized fruit. Sometimes, tomato fruit trusses are also pruned to allow just 6 - 8 fruit to develop on larger types. This results in a larger individual fruit size.

For tomato plants that are getting too tall for an indoor system, the plants are easily shortened at any stage. This involves removing the growing point, after 2 - 3 leaves have formed past the last fruit truss. This essentially `stops’ the plant from growing upwards and all its energy will then be put into the growth and ripening of the fruit already set on the plant. Some tomato growers use a system of `single truss’ production where tomato plants are `stopped’ after they have set one truss of fruit by removing the growing point. These small plants only reach 40 cm in height and are grown at a high density - this makes maximum use of overhead artificial lights while resulting in rapid fruit development and early yields.

Fruiting plants such as tomatoes, peppers, cucumbers and others need to have healthy foliage with a large surface area to produce the sugars required for maximum fruit growth. For this reason removing any healthy foliage from around the fruit is not a good idea. It reduces the food supply to the fruit and can also expose the fruit to high light levels, which lower the quality of the fruit. In tomatoes, the 2 leaves below the fruit truss and the one leaf above it are the main food producers for those fruit, so these should never be removed until after the whole truss of fruit has been harvested. In most fruiting plants, the 3 - 6 leaves directly around the fruit are those that supply assimilates for fruit growth and development and these should always be left on the plant. If a fruiting plant is to be pruned, always leave at least 2 - 3 leaves past the last fruit on the stem or lateral before removing the growing point or excess foliage.

Summary

Pruning and manipulation of plants in an indoor garden is an essential plant maintenance operation, which ensures the best display or largest fruit possible. It is the best way to deal with premature flower formation or excess, dominant shoot growth and produce those compact, uniformly flowering plants that will provide the most attractive garden.