Growing vegetables in a greenhouse

Growing Vegetables in a greenhouse allows you to enjoy flavorful vegetables out of season.  Being able to watch the production is a treat for the soul in the dark days of winter.  Here is a list of things you’ll want to consider before beginning this endeavor:

Plant selection:  There are many vegetable varieties that have been developed specifically for greenhouses.  These varieties often have characteristics of disease resistance, high yields, bolt resistance, and resistance to physiological disorders (I.e., fruit cracking on tomatoes).  Look in specialty seed catalogues and websites.  One company that has many varieties is Johnny’s.

Light and glazing – Glazing is the outside surface that allows light into the structure.  There are many choices including glass, vinyl, fiberglass, polycarbonate, acrylic and polyethylene.  The surface you choose should have a long life expectancy under years of sun exposure, and should be able to withstand hail if your climate has this weather occurrence often.

According to Colorado State University research (as stated in the book referenced below), plants grow better under glazings that are NOT clear.  This is because they scatter light over a wider area resulting in more even distribution of light.

Air Temperature – Plants don’t like temperatures above 85 degrees Fahrenheit.  They will stop growing and may drop flowers and may not set fruit above this temperature.  And 50-60 degrees is the optimal low temperature for most plants or similar problems will occur due to cold.  Of course this is a general rule and there are some exceptions.

Ventilation – It’s vital to keep the heat down on hot sunny days with adequate ventilation.  A system that provides a cross flow of air will keep the temperature at tolerable level for your plants.  Vents can be opened automatically by using commercially available automatic openers.  Here is one that is similar to one that I have used in the past.  Cooling fans are another option.  With both fans and vents, the rule is to place them high so you will get a much more efficient flow of air.

Heating – Relying on the sun for heat works for a good part of the year, but depending on your proximity to the equator, this reliance may not suffice during the dark days of winter.  A supplemental heater will allow you to have more control over winter heat.  Seek expert help for calculating how many BTUs your greenhouse needs based on its size.

Humidity control – Especially in winter when there is little outside air flow, a greenhouse can become quite humid, and plant diseases can proliferate in high humidity.  Humidity can result in condensation which will block light and solar heat on the greenhouse walls.  To control humidity, water early in the day, water only when needed, and use fans or vents for circulation.

Pollination – because of the greenhouse’s closed environment, insect pollinators don’t have access to greenhouse plants.  Plants that are pollinated by weather conditions like wind (tomatoes) also will be a challenge.  There are ways to manually pollinate satisfactorily.  And another solution is to look for parthenocarpic varieties of vegetable seeds which will set fruit without needing pollination.

A Greenhouse Guide

A great resource for growing vegetables (and other plants) in the greenhouse is the book Greenhouse Gardener’s Companion by Shane Smith.  This book has a wide array of helpful tips.  There is a chart with the amount of light required by each vegetable crop, and many suggestions are given for heater options.  There are a plethora of high humidity and ventilation solutions.  The book also has lists of the amount of greenhouse space needed for each vegetable, and has helpful tips like the Swiss chard tip, which I REALLY appreciate: instead of growing spinach which may bolt quickly (like after five weeks) grow Swiss chard which is almost as healthy, and you can cut the leaves and they will regrow for a season long crop.

The book also has many ideas and options for insulating the greenhouse.  One suggestion I take strong exception to.  On page 51 of this book, the author suggests bubble wrap as an insulator from Charley’s Greenhouse Supply.  For our farmers market hoophouses (four of them that were 30 feet long) I purchased and installed this bubble wrap, and it deteriorated within two years.  I then had a huge mess of little plastic bits all over my hoophouses.  I recommend NOT buying this product.  Instead, obtain a solar pool cover which is a type of bubbled plastic, but is much stronger, and lasts much longer.  It can be found at swimming pool suppliers.

If you’d like to dream big, I suggest having a look at this absolutely beautiful greenhouse. Click on the photo for more details.

I hope you are able to enjoy the pleasurable and sustainable benefits of growing in your own food in a greenhouse.