There are many ways to plant fruits and veg in a greenhouse. Many growers switch from growing directly in the ground to using pots and grow bags. However, depending on what you are going to grow and how big your greenhouse is, building a raised bed in a greenhouse can be a great option.
What is a raised bed?
Raised bed also known as garden box is an enclosed unit which is lifted above the ground level. The most common materials for the garden box are wood, rock and concrete. Many growers prefer using raised beds outdoors but you can build them in a greenhouse too.
How to build a raised bed in a greenhouse?
For this project you will need wood boards, spray paint, coated deck screws, drill and pre-mixed soil.
1. Make a layout of raised beds inside greenhouse
Raised beds can be any length and width. For a standard hobby greenhouse which is 6 ft long and 8 ft wide you can build two raised beds on each side of the greenhouse. Leave the space in the middle for walkway, approximately 1 to 2 feet wide. You can line it with gravel or lay the concrete slabs. Just make sure that you don’t leave the native ground as it can dirty and messy.
You can modify the traditional layout a little bit and at the end of greenhouse join two raised beds together. That way you can utilize space and grow more plants.
Another popular option for a bigger greenhouse is to build three raised beds. One on each side and one raised bed sits in the middle. You will have two walkways between them.
I really enjoy growing in raised beds, you don’t need to bend down when planting or weeding. You can even sit on the edge of the raised bed in the middle to work on another one. Or in a smaller greenhouse you can bring the chair, put it on a walkway and sit down.
How wide should a raised bed be?
Generally, a raised bed should be around 2 to 3 feet long so you can easily reach the plants. When building a raised bed in a greenhouse I wouldn’t recommend anything wider than 2ft especially in a small structure. You would need to reach in when planting or looking after your plants. You can go for 3 ft long raised bed but in this case make sure you have walkways on both sides.
What is the best height for raised garden bed?
The best height for a raised garden bed is around 11 to 12 inches. It can depend on the size of the wood boards that you use. The standard raised bed is made out of two 2 x 6 inches wood boards. As a rule of thumb, if your soil is not good enough, you want to make raised bed as high as possible.
Also, the higher it is means that plant roots will have enough space to grow. So, once you decided on the sizes of raised beds, you need to mark where you are going to build them in a greenhouse. The easiest way to lay out the borders is to use spray paint.
2. Choose the materials for raised bed
Generally, wood board is the most common material for a raised bed but you can use stones, concrete blocks and bricks. When choosing wood you can consider rot resistant types of wood like cedar, black cherry, oak , black locust, Osage orange or redwood. Although they last from 10 to 15 years, buying them can be quite pricey.
You can go for a pressure treated lumber you can find at a lumber yard. It is not going to last as long and many growers have concerns regarding the chemicals used to treat the lumber. These might leach into the soil but to prevent it you can lay plastic at the bottom of the raised bed.
3. Assemble the raised bed in a greenhouse
Cut the board to a desired size. Depending on the size of your greenhouse you can use 2″ x 4″, 2″ x 6″, 2″ x 8″ and 2″ x 10″ boards. Keep in mind that if you use 2″ x 8″ boards brought from lumber yard these actually measure 1.5″ x 7.5”. Use two 8 ft long board for the sides (depends on the size of your greenhouse) and 2 ft wide boards.
Build a simple box. Use coated deck screws to screw the corners together. Also, take small 2″ x 4″ board and drive this post in corner and screw boards into the post. Screw from both sides to help secure the corners.
If you are building a long raised bed, cut the board the same length as you want the width of the raised bed to be and screw it in the middle. It will help to hold the whole structure together. Level it out, if it is low on one side – just shim it up a little bit.
How do I keep my raised bed from bowing?
The weight of the soil is going to push out all the time which causes raised bed bowing. Also, because lumber is a flexible material, pressure of the soil will provoke the wood to bow out.
So, to keep raised bed from bowing you need to secure the corners with the posts and you can also set up cross supports every 4 to 6 foot along the length of the bed. Moreover, when building the frame of the raised bed make sure that wood grain on all boards is facing inward.
4. Fill the raised bed with soil
Prior to filling it with soil, you need to prepare native ground. Smooth the ground in the bed and walkway. If you haven’t used the native soil in your greenhouse for planting, you need to dig the soil to a depth of at least 16″. Remove the rocks and watch out for any growth.
You don’t want anything to be growing apart from what you are going to plant. If you leave these roots, they will take up water and nutrients leaving desired plants in deficiency.
What is the best soil mix for raised beds?
Growers prefer to use raised beds in outdoor gardening and greenhouse because of the very good drainage. To achieve that good drainage you need to use pre-mixed soil. Starting with the fresh soil is always a good idea to prevent the growth of weeds.
So, the best soil mix for raised bed in a greenhouse is 30% peat moss, 30% vermiculite and 40% compost. You can amend how many parts of peat moss to use depending on the plants but the key is to have at least 40-50% of compost.
If you want to know what soil types are there and how can you improve your existing soil, I have put together a blog post on the best soil for greenhouse plants.
Can I use garden soil in a raised bed?
Garden soil shouldn’t fill up the whole raised bed. The structure of garden soil on its own is too heavy and you need a good drainage in raised bed. Instead, mix half garden soil and half potting mix for great results.
Can you fill a raised bed with just compost?
Although compost should make half of raised bed soil, filling the entire bed with just compost is not the best option. Compost provides nutrients for plant growth but on its own it is not enough to retain moisture and nutrients, and keep the soil aerated. That is why you need to mix it with peat moss or coconut coir.
Do I need to replace soil in raised bed?
One of the main benefits of raised bed is that once you layout the soil you don’t need to replace it every year. Generally, when the drainage in the soil is good you don’t need to change it. That ensures that soil borne diseases are not present in the soil.
Digging soil in a raised bed most of the time leads to reduction in number of nutrients , although, it makes the soil more aerated. Another drawback of replacing soil in raised bed is that you can enhance weed growth.
What do I put on the bottom of a raised bed?
For the optimal plant growth you need to prepare the bottom of a raised garden bed. If you are building a raised bed on a native soil, you need to take out large rocks and put crushed rock or gravel to ensure good drainage. If you set up a raised bed on a concrete floor inside greenhouse, then you need to install drainage tile under the soil for good drainage.
It is also a good idea to install a layer of galvanized mesh on the bottom of a raised bed. That way you will protect your crops from the burrowing pests like moles.
Do I need to line my raised bed?
You need to line a raised bed if you want to prevent weed growth. Also, lining raised bed with a layer of plastic can stop chemicals from leaching into the soil if you are using pressure treated lumber. It is debatable whether layer of plastic on the bottom of raised bed will prevent wood from rot.
Instead of using plastic liner, I would advised to line the bottom of raised bed with landscape fabric. It will create the weed barrier and still ensures good drainage. If you line raised bed with non-porous plastic, it can retain the excess of water and prevent beneficial insects like worms.
