How to Build
Raised Garden Beds in a Greenhouse
Picking a Location and Digging
- Since the bed would be permanent, the first thing I did was to decide a location within the greenhouse. The west wall worked best in my circumstances.
- I then dug down deep enough to allow a minimum of 12″ of soil to grow in and to have room for a drainage system under that.
Base and Drainage System
The next few things took awhile.
- Since we were in tight time constraints while building the foundation, we didn’t think about drainage until it was too late. Digging a hole under the rock foundation for a drainage pipe wasn’t fun.
- To prevent critters from digging up into the grow area I poured a thin layer of concrete . This didn’t need to be pretty and a thin layer was all that was needed.
- I drilled holes in a 4″ pipe (none on the top side) and a hole to connect the pipe that ran under the foundation to drain outside.
- At each end of the main pipe I placed large rocks.
How to Build Raised Garden Beds in a Greenhouse
How to Build Raised Garden Beds in a Greenhouse
Installing the Walls and Finishing the Drainage System
The walls were next.
- Ripping and cutting rough cut lumber from a local mill, I custom built the bed the way I wanted it. I spent a lot of time drawing out plans until I came up with something I really liked. Don’t forget to reinforce the middle sections to keep the weight of the soil from pushing the sides out.
- Using scraps of wood and some gravels from the driveway I fill in around the pipe before covering it with landscaping fabric. The fabric will help restrict the soil from creeping into the pipe and clogging it up. Another help is to not have any holes on the top side of the pipe.
- I added more gravel on top of the fabric to hold it in place and filled in the space with some more scrap wood (untreated).
How to Build Raised Garden Beds in a Greenhouse
Playing in the Soil
Now to fill in the soil.
- Adding back some the native soil I had removed earlier helped reduce the amount of good quality top soil and compost that was needed. This should only be done if the bed it deep enough to allow at least 10-12 inches of good soil on top.
- TIP: When having layers of soil with different densities, mix them together a few inches thick. This will assist with good drainage by preventing an abrupt line in soil condition.
- Inoculate the new bed with worms! Earthworms and red wigglers aerate and fertilize the soil for free.
- A few inches was left unfilled at the top for adding mulch later.
Plants
Once the bed is built and filled it’s time to add the plants.
- Some plants do not do well growing year-round in a greenhouse but living in zone 5 I wanted permanent raised beds for my zone 8-11 plants.
- I had a bit of fun placing the plants in different spots and thinking about how big they will grow or how often they may need harvesting, etc. Once I was happy with the placement I transplanted them into the bed
- Since a few of the plants I have are trees and I need to keep them dwarfed, I left them in large pots. Doing this helps keep the roots from damaging the foundation down the road and will make it easier for me to uproot them about every year or so to re-pot.
Framing for a Cold Frame
- Some of my plants may grow tall and because I liked how the plans looked when I drew them up, I decided to build a tall cold frame and utilize the full height of the greenhouse wall .
- Since my greenhouse uses a rail system for the bolts, I purchased a few washers and bolts that were the same diameter but longer. I then drilled holes in the studs and used the longer bolts to attach them to the wall. This gave me a solid base to attach the rest of the framing to.
- While building the framework I planned for and took into consideration the addition of polycarbonate panels in the future.
- Lastly I added cross beams at the top. Here are some reasons why:
1. the strips create some shade when the sun is at its zenith
2. can tie strings or rope to the wood strips for attaching vertically growing crops such as tomatoes
3. can provide a top shelf for storage
4. some strips are cut longer to hold hanging baskets
5. and I just think it looks neat
Insulating for the Winter
- The first thing I did to winterize the cold frame was to add insulation to the north wall. This insulation wrapped around both sides by about 18-20 inches and across the top about 22-24 inches. I was able to purchase two sheets of the foam board. From those I calculated the size and shape of each piece to maximize the coverage area with the fewest cuts.
- Over the foam board, I attached silver bubble wrap. This gave added insulation on the north wall and helped reflect sunlight back onto the plants.
- Since I could not afford the polycarbonate panels this year, the rest of the framework got covered with a double layer of plastic.
Here is a link to the one I am using and have been happy with. At the time of this posting however, it is unavailable. Hopefully they will get more in soon.
Disclaimer: This contains affiliate links, which means I make a small commission at no extra cost to you. As an Amazon Associate I earn from qualifying purchases.
Heating and Lights
- I spaced out some LED lights throughout the bed and plugged them into timers. The light could come on about an hour before sun-up, stay on about two hours and then repeat at dusk. I did like leaving a purple light on all night.
- One brooder heat lamp (from our chicken coop) was stationed at the north end and a second one about 2/3rds the way down. I had both of these plugged into the seed mat’s temperature control unit; which I don’t need in the heart of winter. They only turned on when temperature fell below the set amount.
- Another highly useful item I placed inside of the cold frame was a temperature sensor that I could watch from inside the house. This let me know if the temperature got too hot so I could quickly open up the plastic, or too cold and I could close it up or cover things more.
Using the spaces in between the plants is a good place to over-winter cold sensitive potted plants.
Cold Beds
I set up the center of my greenhouse for some cold hardy crops; i.e. cabbage, lettuce, etc.
There was not a lot of room to move around and a few of the boxes were already filled, so doing this by myself took a little time and shuffling things about.
At this point I could not buy any more bags of concrete so I just leveled it for the growing boxes. At a later time I am planning on concreting the whole floor. The main reason for doing that is to keep voles and other critters from digging into the greenhouse. I will have a slight slope so that any water can run towards the cold frame area and flow out of that drain.
I didn’t have enough soil to fill every planter box but that was ok. I placed potted plants for over-wintering in the greenhouse into the empty boxes. Come spring, I can easily move them outside.
Over the boxes I built a very simple frame to hold a sheet of plastic when the temperature fell below zero.
