Greenhouse Management: An Overview

Greenhouse management is the key to successful greenhouse gardening.

Whatever the type of greenhouse you’re managing, and whatever the size, to be successful you’ll need to manage 3 environmental variables.

That’s the challenge of greenhouse growing, but it’s also the freedom of greenhouse growing.

Because as a greenhouse gardener, you’ll have the power to control environmental variables over which you have very little control in your outside garden.

You’ll be free of the environmental constraints placed upon you by Mother Nature, and you’ll be able to use that freedom to grow what you want, when you want.  

Greenhouse Temperature: Some Plants Like it Hot, and Some Like it Cold

One of the primary variables you’ll need to control in greenhouse management is greenhouse temperature.

The greenhouse temperature range you’ll be maintaining, of course, will be determined by what you’re growing. All plants have a comfort range, and managing the greenhouse so that the temperature is maintained in that comfort zone will give you the best performance from those plants.

Greenhouse tomatoes, for instance, are most comfortable when the nighttime lows are in the low 60’s F, and daytime highs in the upper 70’s to low 80’s F.

If the greenhouse temperature warms or cools out of that comfort zone, that doesn’t mean the plants are going to instantly die. But it does mean that there will likely be problems in terms of production.

If the temperature is too warm for greenhouse tomatoes, for example, they’ll still bloom and grow (unless it gets way too hot), but will have problems setting fruit. Excessive heat can also cause cracking of the tomato fruit, along with a number of other fruit quality problems.

If the temperatures consistently fall below the low 60’s F, greenhouse tomatoes can have problems with fruit set. And cosmetic problems such as catfacing and russeting can destroy the beauty of the fruit and, for commercial operations, render the harvest unmarketable.

Disease problems are also more likely to occur when temperatures are too low.

So as a greenhouse gardener, you’ll need to know the temperature range that is preferred by the plants you’re growing, and you’ll need to maintain that range using a combo of heating, ventilation, and evaporative cooling.  

Not Enough Light, Too Bright, or Just Right?

Greenhouse tomatoes like lots of light

The second major environmental variable you’ll be able to control as a greenhouse gardener is the amount of light your plants receive.

If you’re a year round greenhouse grower, you’ll find that at times you’ll have far more light than you need, and at other times, not quite enough.

During the summertime when the sun rides high in the sky, you’ll likely have more sun than you need unless you’re growing tropical plants.

During that time of the year, the seasonably intense sunlight provides far more energy than the plants need for photosynthesis.

The extra light raises heat levels in the greenhouse and causes problems such as sunscald. That’s a greenhouse management problem that is easily fixed, though, by just simply blocking a certain portion of the light from entering the greenhouse.

There are a number of options for blocking that excess light.

There are many styles and thicknesses of shade cloth available (I prefer a white, reflective shade cloth). There are also liquid shade coatings that you can spray onto the exterior of the greenhouse like paint, and then wash off later when you want full sun entering the greenhouse again.

Here in Texas, we usually apply shade cloth (or the spray-on shade compound) to our greenhouses around the 1st of May, and remove the shading around the 1st of October.

In the wintertime, of course, the problem will be reversed.

With the sun low in the sky, the sunlight entering the greenhouse will be a precious commodity. You’ll want to be sure that not a single photon goes to waste.

Covering the floor of the greenhouse with a white ground cover can help. Any sunlight that makes it through the canopy of leaves strikes the white floor of the greenhouse and is reflected back up into the leaves.

But depending upon what you’re growing and where you’re located geographically, during the dimmest days of winter there may simply not be enough light for your plants. If that’s the case, you’ll have the option of providing artificial greenhouse lighting to supplement the feeble sunlight.

Artificial greenhouse lighting can be quite expensive if you’re applying it on a large scale, but it’s also very effective, and can give your plants all the light they need to be fully productive.  

Boy, It Sure Is Sticky In Here!

In a greenhouse full of plants, humidity levels can build up very high, very quickly. And that’s very bad – at least for most plants.

During a warm day when the greenhouse is being ventilated, with fresh air being drawn in from the outside and interior air continuously being exhausted, humidity is rarely a problem.

But on a cold day when ventilation is minimal, all of those respiring plants in the greenhouse can cause humidity levels to soar.

That’s a problem, because most of the plant diseases that can be the most devestating in a greenhouse love conditions of high humidity. In a greenhouse growing tomatoes, for example, botrytis can become rampant in favorable conditions, effectively wiping out every plant in the house. Botrytis, in fact, can be a problem in many greenhouse crops.

High humidity levels can even interfere with the pollination of crops like tomatoes, because the pollen grains become too sticky to be transferred properly from the male to the female parts of the flowers.

Unfortunately, controlling humidity is one of the more challenging aspects of greenhouse management – especially during the cold days of winter when you’re not ventilating much.

One trick you can use on a cold day, though, is to pull in a small amount of the frigid outside air, while exhausting a small amount of the warm, moist interior air. Since cooler air holds less moisture than warmer air, pulling in some cool air has the effect of lowering the relative humidity of the interior greenhouse air.

This greenhouse management technique can be psychologically difficult to utilize, especially if you’re paying an arm and a leg to heat your greenhouse.

Pulling cold air into the greenhouse and exhausting some of that expensive warm air can seem counterproductive. But the technique can be very effective at keeping humidity levels down.

And that’s very important!  

Does Greenhouse Management Sound a Bit Challenging?

It can be!

Especially considering that in addition to managing the greenhouse environment, you still have to attend to the same aspects of plant management that outside growing requires – things like fertilization, irrigation, and such.

But just like anything else, the more you do it, the more instinctive it becomes. After you’ve been running your greenhouse for a while, you’ll know what to do, how to do it, and when to do it.

And once your greenhouse management techniques have been honed to near perfection with hard-earned experience, you’ll be able to keep your plants nice and comfy – a favor they’ll repay with lush, beautiful growth and bountiful harvests.    

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