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Importance of growing media to make plants more vegetative or generative

The importance of a growing media to make plants vegetative or generative

August 10, 2017  By Dr. Mohyuddin Mirza

August 2017 – While I have written about the growing media many times, I feel that there is a general lack of knowledge about the importance and techniques to use growing media design and inputs to change the direction of growth, vegetative or generative.

Vegetable growers need to understand this aspect more than bedding plant and ornamental growers because the crops stay longer in the same growing medium. Vegetable crops need a balance between vegetative and generative growth and we would like to avoid domination of one stage versus the other.

On the other hand, we want leafy vegetables to always be vegetative. For example, lettuce, spinach and many Chinese vegetables should stay vegetative. They are not saleable if they become generative.


Basically, vegetative growth is where plants need more leaves, top growth and roots to establish themselves. Generative growth is where plant starts producing flowers and fruits.

A few basic quality aspects of a growing medium: At this time, there are many soilless growing media used in greenhouse cultivation. Many growing media have added biological agents to provide disease reduction possibilities and improve root health. What we should know about the growing medium we are using should include:

  • Water Holding Capacity (WHC).
  • Air Porosity (AP).
  • Cation Exchange Capacity (CEC).
  • Biological Oxygen Demand (BOD).
  • pH and EC.

You must know at least these characteristics based on what crops you are growing and how long the crop is going to stay in that growing medium.

Growing media and container sizes: Photo 1 shows what is typical in a bedding plants greenhouses in May. The container sizes vary from small plugs to 12″ –14″ hanging baskets. Although they are being grown in the same greenhouse, one can see distinct shade in the middle and under the baskets.


Plugs: If you are growing in plugs then the volume will be very small. Plug sizes vary, so growers must understand how much volume of growing medium they are dealing with. Plugs are difficult to manage especially if one chooses a high porosity growing medium. Fertility management is also very precise in growing such plugs.

Photo 2 was taken of petunia plugs under serious stress. The grower just could not keep up with the pH of the growing medium.

This is iron deficiency due to an alkaline pH. In this case the pH was 6.8. These plants will take a good three weeks or so to turn around. Also notice that these plants are already flowering, meaning that they have become generative. These flowers have to be removed to initiate a vegetative response when such plugs are planted.

Photo 3 is an example where tomatoes in 4” pots became root-bound. This is a very generative tomato plant. The lower leaves are showing signs of high EC in the growing medium.

Once the growing media gets root-bound, they dry out much faster so growers must water them more frequently.

Photo 4 is another example of a pepper basket that is root-bound. Because such plants dry out much faster it results in a generative response. The peppers started turning colour (to red from green) very quickly.

Photo 5 and Photo 6 are examples where attention was not paid to the proper supply of nutrients in the growing medium as needed by the plants.

Photo 5 shows a cucumber plant showing iron/manganese deficiency. These cucumber plants were getting standard bedding plant feed that was not enough for these plants.

Photo 6 is of a bougainvillea showing serious potassium – and likely some magnesium–  deficiency.

I want to emphasize that the choice of a proper growing medium is a good starting point. Paying attention to water holding capacity, air porosity, cation exchange capacity, pH and EC will pay dividends in the form of a good marketable crop.

In hydroponics, there is no solid growing medium. Water is the growing medium. The management of crops will be different than when a solid growing medium is used. How to change plant direction in water based system will be the subject of my writing at a later date.

Dr. Mohyuddin Mirza is an industry consultant,

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