These small greenhouses extend the growing season when managed properly.

I have had a polytunnel for almost seven years now. Where I live it is possible to harvest a few things outdoors during the winter months, but having a polytunnel means that I can grow a much wider variety of crops during that time. Having it also means that I have more success with warm-season crops, especially when the summer is a particularly dull or wet one.

If you have a polytunnel, or are considering one for your garden, you may benefit from some of the lessons I have learned over the years.

No Matter How Large the Polytunnel is, You Will Want More Space

My own polytunnel is relatively small, around 10 feet by 20 feet. We decided from the outset to choose the largest one that we could fit alongside the other planting and features on our property. I would recommend that, when buying or making a greenhouse structure, you opt for one that's as large as you can make it. If you can fit in a larger polytunnel, I am confident that you will find that you could do with more space.

From the beginning, I realized that I would have to be inventive in order to maximize the possible yield from the space. I installed wires between the crop bars across the top of the structure and a trellis to enable vertical growing.

In my second spring of having the polytunnel, I decided to add a hanging shelf where I could place seedlings that had graduated from the windowsills inside. This shelf, made from scrap wood and some leftover plastic from the cover of the structure, was also useful for container growing throughout the summer, and for drying onions, garlic, and other crops later in the year. 

Good Layout and Planning Make All The Difference

I have seen plenty of polytunnels over the years, and I would say that the most common issue is poor layout. Placing a single path down the center of a polytunnel between two beds when it is wider than eight feet can make it very difficult to access the backs of the beds.

In my 10-foot-wide tunnel, I decided on a layout with a bed to each side and a central bed in the middle with narrow paths to either side. The paths are just wide enough to walk down and to bring in the occasional wheelbarrow of organic material. But I deliberately kept them narrow to begin with—and actually made them a little narrower over time to maximize the growing area. 

Access is important, but in an undercover growing area, I do think that many people make the pathways wider than they really need to be.

When I was considering bed layout, I thought not only about access, but also about crop rotation. While I do a four-year rotation in outdoor beds, in the polytunnel I have a three-year rotation, and having three beds keeps things simpler. The crop rotation plan centers around tomatoes (with companions), legumes, and brassicas or leafy greens. I grow plenty of other crops alongside these other groups, but the rotation focuses primarily on these three plant families.

Things Can Look Very Different in a Polytunnel Garden Year-To-Year

One of the things that has most interested me about growing food in a polytunnel is how different it can look and how much things can vary from one year to the next. When I can sow and plant depends on the weather in a given year. Some years, things have been growing strong by early March; other years, things do not really get going until mid- to late April.

The polytunnel does tend to remain frost-free most winters. But once or twice we've had colder conditions and I have had to use extra covers and protection to prevent damage to overwintering crops. 

I have learned that I can temper some of the most extreme temperature fluctuations by adding thermal mass to the space. Stored water and stones absorb the heat during the day and release it slowly when temperatures fall. Before adding extra thermal mass, I did find I had more issues—not just with winter cold, but also with high temperatures in summer.

Polytunnels Offer Some Protection, but Pests Can Still Be a Problem

Polytunnels are fantastic for protecting crops against a range of problems and pests. For example, brassicas in the polytunnel won't be eaten by pigeons. We have a lot of pigeons nesting in a neighboring barn building, so leafy greens outdoors are not safe without some form of cover.

Do not make the mistake, however, of thinking that crops inside a polytunnel are entirely safe from pests. One of the most persistent problems I have is voles and mice. They will quickly devour overwintering plants. Adding covers over their favorites and sprinkling cayenne pepper around vulnerable plants is the only way to stop them from doing too much damage. It is not 100% effective, but it does help.

In summer, keeping the doors open as much as possible, as well as practicing companion planting, allows the polytunnel crops to benefit natural predation in the same way as crops grown in beds outside.

Timing Is Extremely Important

Perhaps the most important lesson I have learned as a polytunnel gardener is just how important timing is. I need to think very carefully about when to sow and plant, taking into account the conditions in a given year. But since I am growing year-round, I also have to make tough timing decisions about when to clear summer crops to enable winter growing. Through trial and error, I have worked out that occasionally it makes sense to remove productive crops in order to make way for overwintering ones, and to maximize yield from the space and make full use of it throughout the year.

Fact checked by Haley Mast