There are so many different crops we can grow in hydroponic systems, it can be difficult to figure out exactly how to balance the growth of our plants so we can maximize the harvestable portion we are after. Sometimes we end up with a lot of plant, and very little of the harvestable parts we are growing the plant for in the first place.

It’s not uncommon, in the highly protected environment of an indoor garden, to experience lush vegetative growth at the expense of fruit yields, and plants that simply take forever to produce the first flower bud, or just don’t flower at all. Sometimes foliage crops such as lettuce start to bolt before they ever reach an edible size.

Herbs grown for edible foliage can also flower early, reducing the lifespan and productivity of the plant, while flowering plants may need certain triggers before any blooms are initiated on otherwise mature, healthy plants. Understanding the physiology of a plant being grown is one way to determine the conditions it needs to be most productive, but to go a little further, there are some advanced techniques that can help steer crops in the right direction.

Vegetative vs. Generative Growth

Commercial growers are usually well aware of the vegetative versus generative growth balance aspects of their crops and how to influence these. Vegetative refers to the production of leaves and stems, and generative refers to the production of flowers, fruits and seeds.

Hydroponic crops generally go through an early vegetative stage (seedlings), on to a fully vegetative phase when the plant develops sufficient foliage to support flowers and fruits, then finally, when conditions are right and the plant is at the correct stage of maturity, to a generative phase of flowering, fruiting and seed development.

While most common hydroponic crops have vegetative and generative stages if grown for long enough—even lettuce will flower and set seed if given enough time—what is important is which phase produces the harvestable part of the plant.

Tomato growers aim for strong flowering, fruit set and fruit growth, while lettuce growers only produce through the vegetative phase and harvest long before the plant enters the reproductive stage of its life cycle. Plants such as tomatoes, peppers and cucumbers also need to have the correct balance: enough foliage to support fruit growth, but not so excessive as to be wasting energy on leaves instead of flower and fruit production.

This control over plant balance is somewhat influenced by genetics, but it is also linked to environmental and root zone conditions, which are under the direct control of growers. We have the ability to help nudge growth in the direction we want, making our indoor gardens more productive and providing the challenge of fully understanding what makes each plant species tick.

Excessive Vegetative Growth

Overly vegetative plants are usually easy to spot. They tend to produce large, lush, lighter-green leaves. Tomatoes and similar crops may have thick stems, often described as tree trunks, and any flowers that develop are often small, weak and may even drop from the plant. The plant canopy is usually dense with many long leaves, fast growth and rapidly increasing plant height.

This sort of excessive vegetative growth is common in young plants and is also partially genetic—some cultivars are naturally much more vegetative than others. Other conditions that favor vegetative growth include substrates that retain a lot of moisture, such as fine-grade coconut fiber; a lower EC; plenty of water from frequent irrigation; use of vigorous rootstocks; and a mild, stress-free environment. Low light, overcrowding, high humidity and excessive warmth can also push a plant in a more vegetative direction.

What to Do About Excessive Vegetative Growth

Growers battling excessive vegetative growth should select cultivars described as having a generative growth habit. These exist for tomatoes, peppers, cucumbers and an increasing range of other flower and fruiting plants. The main tools used to steer plants in a more generative direction are temperature, CO2, high light, reduced moisture and increased nutrient (EC) levels. CO2 enrichment favors generative growth once flowering and fruit set has occurred.

A more advanced tool is the use of day/night temperature differentials (DIFs), which can be used to steer a crop back into more balanced growth. DIFs are widely used in commercial crop production, but they work just as well in indoor gardens where the grower has good control over temperatures. High daytime temperatures tend to promote plant stretching as well as an increase in leaf area.

Night temperatures do not play a role in this, but the difference between day and night temperatures is important for flower and fruit development. Environmental conditions also play a role. Low light combined with warm temperatures results in stem elongation and a tall, weak plant, so temperatures should be matched to the light levels.

Keeping day/night temperatures similar stimulates vegetative growth, while a greater DIF (night temperature much lower than the day temperature) results in more generative growth. It’s a good idea to run day/night temperatures that 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 fruit set. From then onwards, tomato and capsicum growers may change the DIF settings to push either vegetative or generative growth as required.

Stressing plants out slightly by using a higher EC and deficit irrigation practices will also have a generative effect. Deficit irrigation practices include reducing the volume of water applied at each irrigation, allowing more time between watering and allowing the media to dry out slightly overnight by restricting early morning and evening irrigations. This method must be used with caution, as moisture fluctuations in the root zone can lead to an increase in fruit splitting and cracking, and it is also associated with an increase in blossom-end rot under certain growing conditions.

Flowering Triggers and Forcing

For many plant species, flower formation happens when the plant is mature enough and growing conditions are right for this to occur. However, there are exceptions. Some commonly grown species have day-length requirements (photoperiodism) or require a period of exposure to cold (vernalization) before they will initiate flower buds.

For example, spinach requires a long photoperiod (day length) to initiate flowers, but rice requires a short day length. Day-neutral plants such as cucumbers and tomatoes do not initiate flowers based on day length, but flower once they reach a certain size or age, or in response to other environmental conditions such as temperature. Strawberries can be either short-day, long-day or day-neutral.

To confuse matters further, some plants need certain combinations of day length, temperature, plant maturity and environmental conditions for flowering. Furthermore, the number of flowers and strength of blooms, pollination and fruit set are also influenced by a wide range of different factors both within the plant and the growing environment.

Flowering cannot be initiated by simply switching to a bloom nutrient formation. Bloom nutrients are specifically designed to provide the different ratio of elements required for flower and fruit tissue development, not to force a plant into flowering when it is not physiologically ready to do so.

Unwanted Flowering

Crops grown for their vegetative parts, such as lettuce and herbs, can flower prematurely. Typically this occurs most rapidly under high light, long days and warm conditions, especially with cool-season crops like lettuce, spinach, endives and radishes. Basil is also prone to early flowering in hot summer growing conditions, limiting the amount of leaves that can be harvested.

In overly warm climates, chilling the nutrient solution is an effective way of delaying the flowering phase, allowing the plants to grow enough foliage for harvest. Other methods include shading plants to reduce temperatures, growing slow-bolt cultivars, avoiding overcrowding and cooling the growing area.

Fruit Upsizing

Small, undersized flowers and fruits are a common complaint, particularly amongst tomato growers. Sizing up slow-growing fruits is based on the fact that the warm parts of the plant (buds, leaves and fruits) attract more sugars than cooler parts. A small difference in temperature can make a considerable difference in the distribution of sugars within plants, and warm fruits attract more sugars for growth.

The optimal fruit tissue temperature for importing sugars and upsizing a tomato fruit is around 73-77 oF. Growers use a technique called a pre-night drop to help boost fruit size. This process involves increasing the heat in the late afternoon, which is absorbed by the fruit tissue, while sugars accumulate in the photosynthesizing leaves.

In the evening, the temperature is dropped back down quickly by several degrees to 60-62 oF, which causes the thinner leaves at the top of the plant to cool quickly, while the larger fruit mass remains warmer for much longer. The cooler leaves then unload their sugars on the warmer fruit, forcing more sugars for growth out of the foliage and into the fruit. While this technique is most widely used on tomatoes, it can be used on many different types of crops.

Understanding the triggers and tools that can be used to balance or steer plants into the growth pattern we want can be extremely useful. The strategy is widely used by commercial gardeners.

An indoor garden is the ideal environment to experiment with such techniques, as precise control over photoperiod, temperature, moisture and EC can all be achieved. Since different species can vary considerably in their requirements and response to flowering triggers, it pays to know a little about the physiology of your hydroponic crop, and how to get the best from any new techniques.