Greenhouse Canada

Features Crop Culture Inputs
Growing In The Green: January 2007

January 21, 2008
By Melhem Sawaya


Timely pre-season tune-up will help prevent plug production problems. With such a narrow window of opportunity to get off to a good start, an ounce of prevention is indeed worth a pound of cure.

During preparations for every seasonal crop, a series of steps are required before the actual production begins. These steps are commonly called the “tune-up.”

We take our cars in for a checkup before a long trip, or during intervals of every so many kilometres. Athletes check their equipment before a match, whether it’s their rackets, bows or skates, etc.


I worked on a farm for a couple of years and my boss, Mr. Freeman, would always stress the need to get the machinery well-tuned well ahead of field work for two reasons:

• It takes much longer to fix machinery after it breaks than a preventive tune-up requires, not to mention how inconvenient and frustrating it is to deal with equipment that has broken down when you need it most.
• Time and schedules must be met or the crop will suffer.

Even in field production, you have a narrow window of opportunity for planting or harvesting. At the farm I worked at, there was a very narrow window for planting. One day it might be too wet, then next day would be perfect, and third day could be too dry. (You guessed it, we worked on clay fields!)

But the timing window for plug production is just as narrow, and the preparations should be completed well in advance. Some of the things to check include the seeder, the germination chamber, the mist lines, the flat fillers, and the seed storage area (hopefully you have one by now and are not storing seeds in your desk!). You should also conduct a practice run with some seed to check the entire process. And, most importantly, check to ensure your staff is ready and well trained.

The most important aspect of greenhouse profitability is having no shrinkage! This is not just about heating costs, labour efficiency, or low prices, though these would all be factors of profitability. But compared to shrinkage, they are minor costs.

So, here are some points that could help with your plug production tune-up, and during the actual production.

•    The storage of seeds is very important. Do not open the container until you are ready to sow, and hopefully you will use them all. If you don’t need a large order, use several smaller packages.
•    Using leftover seed from the previous year is often a waste of time. Sow a small sample of it to give you a good idea of whether it is viable or not. Some seed stores better than others.

•    For optimum seed germination and growth, a proper pH of 5.7 to 6.2 is a must.
•    Soluble salts should be in the 0.4 to 0.8 range, a 2:1 dilution.
•    Sodium and chloride levels should each be less than 40 ppm.
•    Phosphorous levels should be in the 8-10 ppm range or else the plugs will be completely stunted.
•    Good holding capacity is essential. Testing the media in-house and at a lab – before any seeding is done – to see if the pH, soluble salts and other nutrients are within range is the most important factor of plug production. About 90 per cent of plug problems are related to pH and soluble salt issues.

•    The physical properties of the plug flat are also important. The deeper the cell, the better it is for drainage due to the deeper columns.
•    Filling the flats evenly with the same density is also important to ensure even moisture levels during the plug-growing period.
•    The soil should be damp to the touch but not dripping wet. One way to check moisture content is by taking a handful of the mix after wetting, and then squeezing it. If it drips, it is too wet. If it crumbles, it is too dry. But if it stays in one clump, then it is the correct level of wetness.

•    Adjust your seeder so that there are no misses. Perhaps it is time to change nozzles, check the drum, or have a simple tune-up.
•    Pay attention to seed placement. Seeds placed near the edge of the cavity could desiccate, especially when we are trying to run the crop on the drier side.
•    If you are covering seed, never use fine vermiculite because the idea is to create a micro-environment of high humidity, not to bury the seed. Use of medium vermiculite of proportionally sized particles is recommended. Do not use the dusty leftover vermiculite.
•    If plug flats are passed through a watering tunnel, make sure the flats are evenly watered with gentle irrigation so the seed is not buried. This is especially important with very small seed.
•    Sown plug flats should not sit in the growing area for hours, because the minute the germination process starts, it should not be interrupted by a drying period. It is especially important to sow the plug flats and have them moved into the germination chamber or propagation area to receive the humidity or water cycles as required.
•    Sown plug flats should be transported to the propagation area without being severely shaken, as this might bury the seeds more than is desirable.

•    Plugs need to be fertilized at early stages, but at a low EC level of .75 mmhos of total water and fertilizer. This is a recommended starting point, assuming we are starting with a low EC plug media.
•    Avoid the use of high ammonium fertilizers, especially early in the season when light levels are low. As well, the use of ammonium fertilizers is not recommended if you are trying to cool the plugs.
•    Many plug nutrition articles recommend to not fertilize with high phosphorous levels. The emphasis with this recommendation should be on the “high” rather than the “not.” Maintaining low levels of phosphorous in plug media is as important as any other element. A phosphorous level of 8-10 ppm in the plug media is important in the energy compounds, such as ATP, that the plant needs for synthesis and degradation. At the same time, high levels of phosphorous will cause plug stretch and inhibit the absorption of copper, iron, zinc and boron.
In simple words, moderation is the key. Test your media frequently to know your levels.

Adjust the environment and crop treatments to support every plug stage. Do not treat all stages and all seeds the same because they are not. The KISS (Keep It Simple Silly) theory has limits before negative consequences occur.
Many methods can be used to regulate growth, but with our low light levels early in the season, we depend more on chemical growth regulators. Pick the right growth regulator and the right rate to use on your plugs.
The rate and type of growth regulator to use varies with:

•    Temperature: higher temperatures require higher rates.
•    Light levels: The higher the light levels, the less growth regulators are needed, especially if that crop flowers with accumulated light levels, i.e., geraniums.
•    The type of grower you are: wet or dry.
•    Greenhouse environment: glass vs. poly.
•    Humidity: greenhouses with high humidity would have more need for growth regulators.
•    Cultivars within the same genus require different rates, i.e., red vs. purple petunia.
•    Varietal selection will determine the rate of growth regulator to use, i.e., Fantasy petunia vs. Wave petunia.
•    The rate changes with a grower’s habit of applying the growth regulators, i.e., light vs. heavy spraying: that’s why we should spray with a predetermined volume per area.
•    Plant condition when spraying: if the leaves are wet when we are spraying, then the rate is much less effective, if there is any effect.

With all these variables in mind, the only way to know what rates to use is by doing your own trials based on recommended rates, and judging for yourself.

Note: Many of the label recommendations are based on very hot and sunny environment so the rates will be too high for Canada and northern states.

Melhem Sawaya of Focus Greenhouse Management is a consultant and research coordinator to the horticultural industry. Comments on this or any other article are always welcome by e-mailing him at

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