How to Turn a Raised Bed into a Greenhouse

When it comes to season extension, you have plenty of options. You can cover your plants with row cover to fend off frosts or try growing them indoors. But if you want to keep growing outdoors and need serious cold protection, converting a raised bed into an insulated greenhouse or hoop house is a great option.

While greenhouses may sound complicated, all you need to transform your bed is a few spare hours and some easy-to-find materials. I’m talking about things like PVC pipe, greenhouse plastic, and screws. This covered bed will be better able to withstand cold and wind, allowing you to start crops earlier in the spring and grow later into the fall.

Design Your Greenhouse

Before you start transforming your raised bed into a greenhouse or low tunnel, you need to decide on a design. If you search the internet or ask your gardening friends for suggestions, you’ll likely be met with a handful of options. And while one design might be best for your neighbor down the street, it may not be the best option for you and your garden. Therefore, take a few minutes to think about the following elements of your design.

Height

You can make your DIY greenhouse as tall or as short as you’d like, but keep the pros and cons of different heights in mind. Tall arches can accommodate large plants like tomatoes, broccoli, and cucumbers, so you don’t have to worry about plants bursting through your plastic. Larger greenhouses heat up slower than smaller greenhouses, but they also lose heat more slowly. That means larger greenhouses hold heat longer after the heat sets.

A downside of building taller arches is the increased cost of materials. Not only do you need longer arches, but you’ll also need to obtain a larger sheet of greenhouse plastic. If cost is your main concern, stick with a shorter height.

Access

While it’s easy to imagine setting up your greenhouse in the fall and not touching it until spring, this is unheard of! A greenhouse is that it allows you to extend the growing season in both the fall and spring. Therefore, you have to be able to reach into your beds to harvest and maintain your crops.

With this in mind, think about how you’d like to access your bed. Do you reach into your bed from one, two, three, or four sides? Can you work in the soil if you roll plastic a foot off the ground, or do you prefer five feet of open space above the beds? Noting your preferences will help you create a design you’re happy with.

Snow

Different greenhouse shapes handle snowfall differently, so think about how much snow your area receives. While any well-executed design can handle an inch or two of snow with ease, a foot of snow is a different story.

If you expect to receive multiple heavy snowfalls throughout the winter, consider a gothic design. This shape features a peaked roof that helps shed snow. The rounded tops of quonset-style designs can’t shed snow as well, so they’re not the best option for snowy areas. However, if you don’t mind brushing off the snow during a storm, any style will do!

When it comes to snow, you may need a more durable design than a hoop house made with PVC and greenhouse plastic. A repurposed window or sheets of polycarbonate may be better for gardeners who expect multiple inches of snow in a season.

Two Main Options: Rounded and Peaked Roofs

Although there are hundreds of different ways to add a covered structure, the most common method is using PVC pipe. This material is readily available, affordable, and easy to maneuver, so I’ll cover this design in this article.

If you decide to use PVC to construct your greenhouse, you’ll need to choose between two main designs. The first option involves bending PVC pipes to form rounded arches. This option is the easiest of the two and works well in many instances. The other design involves assembling multiple straight pieces of PVC to form a peaked roof and straight sides.

For the purposes of this article, I’ll cover how to design and install a greenhouse with rounded arches.

Hoop Spacing

Regardless of which design you go with, you should space your hoops three to four feet apart. If you’re working with a six-foot-long bed, put one hoop at either end and another in the middle. If you have an eight-foot-long bed, you can place either one or two hoops in the middle.

End Wall Design

The tunnel’s end walls, or the short sides, are another design element to consider. You can choose from many different designs ranging from super simple to complex. Thinking about cost, available space, and ease of access can help you decide on a design that’s right for you.

One option is to cover the end walls of your tunnels with plastic. If you opt for this option, you’ll enter your raised beds by lifting up the plastic from the sides rather than the ends. This method works well if you’re looking for a neat design or working with limited space.

Another option is to simply bundle the ends of your plastic at the ends of your beds and weigh them to the ground. This allows you to easily remove the plastic for your entire bed, which is great for airflow and easy access. This design isn’t as neat and takes up more space, but it is easier to set up and less expensive.

Gather Your Materials

Once you determine your design, it’s time to gather your materials. If you opt for a PVC greenhouse, you’ll need the following.

The length of the PVC pipe depends on the width of your current bed and your ideal greenhouse height. You can use this table to help select the correct length of PVC for your situation. 

Most hardware stores will offer to cut PVC in-store, or you can cut it at home using a hacksaw, circular saw, or pipe cutter.

Install the Arches

Once you have all your materials ready, it’s time to start the installation process! Begin by measuring the length of your raised bed and marking the outside of the bed where you plan to place your arches. Remember that you should place an arch every three to four feet. Mark each side of the bed to ensure that the arches go in straight.

After you mark the arches, lay out your materials. Each arch will consist of one piece of PVC, and you’ll attach each side of the pipe to a wooden bed with a pipe strap. Two sets of hands make the installation process easier, but you can complete the job on your own, especially if you have a speed clamp handy.

Begin by lining up the PVC pipe with one of the marks you drew. The pipe should be horizontal, and the end should be on the outside of the bed a few inches below the top of the bed. Hold the pipe in place, then fasten it with a pipe strap. If the pipe moves a bit, don’t worry. You can always loosen the strap and rearrange the PVC so it’s in the right spot.

Once one end of the PVC is securely fastened, move to the opposite side and complete the same process. When you’re finished, the PVC should form an arch and remain in place when gently wiggling it. Complete this process to install the remaining pieces of PVC.

Brick or Concrete Raised Bed? No Problem!

While you can easily drill into wooden and metal beds, installing pipe straps onto brick or concrete is more challenging. If you can’t (or don’t want to) drill into your raised bed, you can secure your PVC arches another way.

One alternative is inserting rebar into the ground right outside your bed. The rebar should be at least a foot in the ground with eight inches above the ground. Once the rebar is secure, you can slide the PVC over the bars until the pipes are flush with the ground.

Optional Step: Install a Center Purlin

If you live in an area with high winds or high snowfall, it’s worth thinking about installing a center purlin. This extra layer of support helps keep your new greenhouse in place when winds start to whip and heavy snow falls. However, it isn’t necessary in most instances, especially if you’ve opted for materials more durable than greenhouse plastic.

A center purlin is a line of PVC that runs across the top of the hoops. It holds the hoops in place and prevents them from bowing in or out, which keeps the greenhouse from collapsing.

If you’d like to install a center purlin, obtain a piece of 1/2-inch PVC that’s the length of your raised bed. Place the PVC under the tops of the arches and attach it to each arch with a pipe strap. Another option is to insert a screw through the top of the arch and into the center purlin.

Add the Covering

Once your arches are installed, it’s time to add your covering! In the vast majority of cases, I prefer to use six mil UV-resistant plastic specifically designed for use in greenhouses and high tunnels. This plastic is manufactured to hold up to sun and precipitation without degrading. Plus, it transmits over 90% of light, ensuring that your plants receive the light they need.

In most cases, you’ll have to purchase a precut piece of greenhouse plastic and then trim it to meet your needs. If you’re between two different sizes of plastic, always go with the bigger option. While it will cost more upfront, it’s easier to work with too much plastic rather than not enough.

When you’re ready to install your plastic, wait until a day with low to no wind. While nothing’s stopping you from working on a windy day, don’t be surprised if your sheet of plastic quickly turns into a giant sail!

Start by opening the plastic into a large, flat sheet. Pull one side of the plastic over the PVC hoops until it is sitting in the middle of the greenhouse. Grab a pipe clamp and use it to fasten the plastic to the top of the middle hoop. Work your way down, pulling the plastic taut and adding a clamp to the top of the next hoop. Complete this process on the other side.

Once the plastic is secured to the tops of the hoops, it’s time to secure it to the bottom. One option is to use another set of pipe clamps to secure the plastic to the bottom of each PVC hoop. Another option is to leave the side of the plastic long so they reach the ground. You can then add rocks, bricks, sandbags, or other heavy objects to secure the ends of the plastic to the ground.

Now comes the end walls. If you want to go the simple route, grab the extra plastic at the ends and secure it to the ground with a heavy weight. Trim any access plastic to avoid it blowing in the wind. You can also bundle the end of the plastic and tie it with a rope. Drive a t-post into the ground and tie the rope around the post to secure the plastic in place.

If you want to create a more permanent end wall, secure a piece of plastic so it’s flat against the end arch. Secure the plastic to the arch using pipe clamps. Complete this process on the other end of the tunnel.

Ensure Security and Easy Access

Once your greenhouse is all set, it’s time to make sure it’s ready for long-term use. Gently shake the tunnel and look for any wobbly pipes or loose pieces of plastic. If you notice problems, tighten the screws on the pipe straps and/or add some extra clamps to hold the plastic onto the PVC. You may also need to add heavier weights to secure the plastic to the ground.

Next, check to see how easy it is to lift the sides of the plastic. Small greenhouses will quickly heat up on sunny days, so venting is key to prevent overheating and the buildup of humidity. If it takes more than a few minutes to open the plastic, make adjustments to simplify the process.

You should also keep an eye on your new tunnel during windy and rainy days. If the plastic collapses or detaches from the PVC, add a few extra security measures to keep it in place.