Greenhouse Canada

Growing Media Inputs
Growing Points: The roots of success

February 12, 2013
By Dr. Mohyuddin Mirza


Here we are in the month of February and bedding plant and ornamental
production greenhouses are buzzing with activity. Plugs are arriving and
being transplanted.

Here we are in the month of February and bedding plant and ornamental production greenhouses are buzzing with activity. Plugs are arriving and being transplanted.

This month came so quick after poinsettias in December!


One poinsettia grower I worked closely with started using calcium nitrate immediately after transplanting the cuttings. The results were very impressive in terms of root growth and structure. I saw so many healthy roots that directly translated into thick branches with absolutely no breakage.

I checked the roots a few months later and they looked quite healthy. Indeed, overall plant health remained quite exceptional. That is what you see in the picture below. The point I want to make is that focusing on early root growth will result in a better final product for marketing.


Remember also that most of the commonly used fertilizers, such as 20-20-20, 20-10-20, 20-8-20, don’t have any calcium. Many bedding plants growers don’t add calcium-based fertilizers with their regular “complete” fertilizers.

Check the May 2012 issue of Greenhouse Canada for details under the article “Fertilizers and Fear Factor.”

(Features and past issues are posted to .)


■ It would not be wrong to call February the month for plugs and roots. Good roots are the foundation of the entire plant and root health has to be maintained constantly during the production cycle of the crop.

Much attention has been paid over the past decade to developing growing media with different porosities and with added biological amendments. They focus on root health and rooting and there are claims of more root development and therefore better plants.

I would like to share with you some examples of roots I have seen. It may help to rationalize what is good and what is not so good. Here are a few examples I’d like to share with you.


The first picture is of a “tri-plug” received in late February from a reputable supplier. This plug consists of three plants and there are many names suppliers use to brand their products. Three different plants are used and it makes it easier to plant them. I found that each plant has different types of roots and they will maintain their type of roots when plants are “grown up.”

The picture on the right shows how the plant will look in April. Roots have to be managed carefully in the case of these plugs. Knock these pots out and check how roots are behaving in terms of growth. Plants with stronger roots may overpower the one with smaller or poor root systems. Growers of these tri-plugs have to develop information on which plants to combine in terms of their root structures and pH requirement of plants.

These dracena plug pictures, below, tell a different story.


They are slow-growing plants and quite often you will see roots that have gone down and started a ring around the base.

The reason for this type of root development is the picture on the right hand side. Notice a thick mat of “scum” on the surface. This means oxygen is cut off from the top and even water cannot penetrate through the surface.

Water goes down from the sides and accumulates at the bottom and that is where the roots are. These roots then go round and round and the grower will have to be careful during transplanting.

If these plugs are planted as such, then there will be a delay of rooting for at least a week and that may cause tip burn on the leaves.

I want you to look at these three pictures as well.


These are begonia plugs that arrived in February. My thumb sinks in when I apply pressure. The plant is desperate to survive as it tries to produce a few roots.

Growers will sometimes transplant such plugs because the colours are needed. I think they should be thrown out and the supplier notified. (The plug also smelled rotten.)

I followed up last year and you see the results (bottom) – no roots, a tuber has been set due to stress on the plant, and the plant never gained any new growth. It just barely maintained itself until it was thrown out.

The cost of this mistake was the price of plugs, transplanting costs, growing media and pots, heating costs, fertilizer and water costs, any fungicides used, and of course the labour costs. Paying attention to roots would have prevented these losses.


■ The picture above left is of cucumbers in coir (coco fibre). Featured are nice, healthy, white roots … not so thick, but with lots of root hair. These types of roots are associated with very high production. In this case, the grower achieved over 250 cucumbers per square metre.

The middle picture is an aquaponics system in which tomatoes are being grown in a fish-based system. Nutrient levels are so low, the plants are producing more roots to capture those nutrients.

The picture at right is a classic example of a rockwool cube left too long on the plastic, with roots forming a horizontal mat. Once placed on coir, it will take about four days before the roots get established in the growing medium.

Here are few more facts to consider:

  • Roots grow continously and need root hair for water and nutrient absorption. Roots need a food supply from the leaves to grow. This supply of food depends on a 24-hour-average temperature.


■ Plants grown under lower 24-hour-average temperatures will grow in a vegetative direction and there will be more and thicker roots. Day temperatures of over 25 C in the rootzone cause stress due to high respiration, and leaf related spotting can be common.

  • For vegetable growers, it is important to understand that plants need to establish a good root system in January and February.

In the case of cucumbers, growers shouldn’t allow any side shoots and fruits to develop anywhere from five to nine leaves. Remove the side shoots and flowers/fruits early and more food will be channelled to the roots.

If tomato plants are too generative, the entire first cluster can be removed to make sure more roots develop.

Similarly in the case of peppers, remove the early flowers to develop better roots.

  • Proper temperature for rooting is essential and growers should know the optimal rooting temperatures. Generally speaking, warmer temperatures promote better roots. That is why bottom heat is used for rooting cuttings.


■ Another point of interest is that if the roots are warm and the top of the plant is cold, then edema will occur due to root pressure. This can easily happen when night temperatures drop too rapidly. In case you have not seen edema, here is a picture of cucumber leaves with edema. It is more common in February.


Smaller plugs get root-bound quickly. Once root compaction occurs, the plant sends a generative signal and flowering starts.

This is why timing to receive plugs is important. Sometimes things come up and growers hold the plugs a week or two longer than they had planned. Plugs become generative and if planted as such the rooting will be delayed for a few days. New roots have to be produced by the plant for absorption of water and nutrients and this delay can cause root rot as well.

  • Remember that roots function best when there is trans-piration – that is, a loss of water from the leaves – and that depends upon a moisture deficit between the leaves and the surrounding air.


■ Simply stated, if the relative humidity is very high (as it is on cloudy days), then the roots are going to be “sluggish.” Because they don’t need to absorb water, they will move it to the leaves. Management of watering under those conditions is important.

  • Nutrients, such as phosphorus, are known to help with root growth, and so are calcium and humic acids. The usefulness of a group of fungi mycorrhiza in growing roots is well documented.
  • The role of rootzone electrical conductivity (EC) should also be understood in irrigation management.
Attention paid to root development early in the crop cycle always pays off at harvest time.


Under low natural light conditions, higher EC is used to control the top growth of vegetable crops. EC values above 4 mmhos cause an osmotic pressure on roots and they absorb less water. This practice was developed when soilless growing media became popular because it is difficult to restrict water applications. In summer, lower EC values are used to manage the vegetative or generative growth of the plants and it is directly related to roots.

If you focus on roots in February, then expect good plants in April and May. Have happy and healthy roots.

Dr. Mohyuddin Mirza is a greenhouse consultant. •

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