Growing Points: The roots of a great crop

July 22, 2013
Written by Dr. Mohyuddin Mirza
Late July and August is the time when rooted cuttings start arriving in greenhouses.

Those growers who handle bedding plants are tired after a long season, and I have seen some neglect in handling the newly arriving rooted or unrooted cuttings.

If you bring in unrooted cuttings, then a proper rooting setup has to be in place and proper misting schedules are critical. Unrooted cuttings have a callus developed but there are no active roots to support the transpiration demands of leaves. That is why under-misting or over-misting can be a problem. The base of the stem can rot very easily with over-misting.

With rooted cuttings, many times they are delivered over the weekend and it is pretty hot in the greenhouse. The boxes are not opened right away, and only a few hours under these conditions are enough to cause root damage and subsequent root issues. Relative humidity is very low in some parts of Canada and roots can be easily damaged. Look at the pictures below:

These pictures were taken on Sept. 18, 2012. It is obvious that the roots got rotted and moved little out of the plug. Fungus pythium was found on the roots and there was also blackening of the stem. This disease is commonly called “Black Leg,” which is an advanced stage of pythium root rot. This fungus attacks the roots first and then moves up the stem. Once the stem is blackened, the plant is dead and no fungicide application will save it.

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With these pictures taken in mid-September, it shows that root problems started developing early. Normally, pythium root rot is an issue when plants are cooled down in late October. The appearance of this fungus disease this early shows that the grower had shut the heating system off and the nights were fairly cool. Waterlogged conditions arose.

Early care of the seedlings: The focus should be on making the plant vegetative and establishing it as quickly as possible. Good root development is the key objective.

Once the rooted cuttings arrive, they should be unpacked immediately in a shaded area of the greenhouse and immediately transplanted in a well-moistened growing medium. I have seen growers let the rooted cutting sit on top of the growing medium for over 30 minutes and that can damage root hairs and young roots.

The shrivelling of roots sends a generative signal to the plant and thus stress is created. Once these root-damaged seedlings are planted in a growing medium, they undergo a shock and a delay in rooting occurs. Growers often don’t adjust watering and fertilizing practices accordingly. High ammonium-based fertilizers should be avoided until roots have moved well into the growing medium.

Electrical conductivity (EC) can rise very quickly due to a large amount of fertilized water being applied when the plants are not well established. Water is moved through evaporation from leaves (transpiration), which depends on roots and surface evaporation from the growing medium. I have seen EC as high as 5.0 mmhos/cm in early stages and that is detrimental to proper rooting. Fertilization depends on the EC of the starting growing medium.

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Main care of the plants: It should be remembered that poinsettias are light-duration dependent (photo period) plants. The leaves (bracts) turn colour when the natural light starts going below 12 hours and that happens around Sept. 21 in many regions. It means that every effort should be made to make plants vegetative in nature, having more good quality leaves and balanced internodes. Proper day and night temperatures, along with the proper 24-hour average temperature, are critical to achieve this vegetative growth.

If you are growing poinsettias for the “real” Christmas market, then you want them to be ready for shipping in early December. That is not the case most of the time now. Poinsettias start coming into the market in the middle of November and this is done by reducing the light duration to about 11 hours in late September through the use of black shade cloth or other blackout systems.

This puts a stress on root development as well. Watch your watering and fertilizing during this time to get a good quality plant for sale.

These pictures were taken in late January 2013 to show how important it is to pay attention to root development in August.

In an effort to grow a large number of poinsettias per unit area, I notice that plants touch each other and spacing is based on a number of pots per square foot. This is based on the number of pots, not on how the plants will touch each other. This overcrowding results in weaker plants and poor roots to support the plant.

I have seen plants literally “crashing” right after Christmas. You can see the top growth of the plants and amount of roots. This is an example of good roots in December when plants are being shipped. This plant has mostly good white roots, the functional roots. A few yellowish/off-brown roots are normal.

"It's important to space out early so they don't touch each other. Pay attention to root growth early. Top-heavy plants have very poor shelf life."


Dr. Mohyuddin Mirza is a greenhouse consultant. • This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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