The Greenhouse Floor
By Lars Lund
It's often said that the lawn represents the floor of the garden. However, having a floor of grass in the greenhouse might not be the most practical choice. But does the greenhouse even need a floor beyond the soil it stands on? That depends on how you use your greenhouse.
Greenhouses have evolved from being solely used for crop cultivation to spaces where people relax and unwind. In the past, greenhouses where hidden in the garden. Today, however, greenhouses are placed more central as outdoor and indoor life merge more and more. Simultaneously, the greenhouse structures have increased in size, both in terms of area and height. Capillary boxes were invented, allowing for a flexible greenhouse interior.
The choice of the greenhouse floor also reflects different personal perspectives. Some greenhouse enthusiasts would never consider cultivating plants in plastic bags or raised beds. They prefer working with garden soil. In fixed beds, drip irrigation can be a great help to ease the watering process, and the roots usually develop better due to the available space, enabling them to reach deeper for the earth's beneficial minerals. This leads to fewer plant diseases and tastier crops.
Whether the cultivation in fixed beds is the better choice in general, is debatable, but it provides the best conditions for the plants. However, when choosing the greenhouse floor, there are other aspects to consider. It is a good idea to reflect upon your preferred cultivation method and the number of plants you wish to accommodate as well as other aspects that might influence your choice.
If your primary reason for having a greenhouse is to create a cosy garden space with a few decorative plants and a fireplace, choosing a floor with a solid surface is the obvious choice. Having a solid floor in the greenhouse does not necessarily mean sacrificing the option of cultivating many plants. You can cultivate plants in capillary boxes and pots, containers, or buckets. Additionally, you can leave a few holes in the ground, where you can for example plant an olive tree, a fig tree, or plants that can withstand winter conditions.
Tiles, Bricks, and Paving Stones
The classic flooring in a greenhouse consists of tiles, concrete paving stones, or bricks. Decorative stones can be laid to enhance the appearance of cement tiles, or cobblestones can be placed around each tile.
Bricks are perhaps the most preferred flooring choice. The bricks can be arranged using different technics to achieve various looks and designs. Often, half-brick or herringbone patterns are preferred. The stones can be laid flat or on edge. The latter provides a stronger flooring but requires more stones per square meter.
The Installation Process
The most significant task is not laying the stones but the groundwork. As with many other projects, preparation is crucial. The substrate must be stable and even.
- First, remove the topsoil layer. Then prepare the ground by using a plate compactor, which can be rented from a hardware store, or a garden roller.
- For a floor without heavy loads, it's optimal to lay 15-20 cm of stabilized gravel without clay. Again, the plate compactor is used.
- Finally, use 3-4 cm of grit sand with a particle size of 1-8 mm, upon which the bricks are laid. Avoid a thicker layer of sand, as it can result in an uneven floor.
Avoid Using Landscape Fabric
It might be tempting to use landscape fabric under the bricks, but this causes the stones to shift and does usually not actually prevent the growth of weed between the bricks. Remember that you will need about 20% more gravel and sand than calculated, as they shrink when compressed.
After laying the flooring, fill the joints by sweeping dry sand into them. If using clay bricks, make sure to use stone dust or weed-resistant sand intended for clay bricks, otherwise it might cause discolorations.
Another piece of advice is to position the stones upright alongside the edges. This will prevent shifting, especially if the stones are positioned under the base.
Using hard-fired stones or what is called “marine clay tiles” results in minimal maintenance. Ordinary bricks absorb more moisture, leading to algae growth.
If you wish to instal concrete tiles you can follow the same instructions. If you desire a different surface on top of the tiles, laying beach sand can create a unique ambiance. Today, there is also a wide range of decorative plastic carpets available.
An alternative solution is a wooden floor. The planks should be suitable for outdoor use, either pressure-treated timber or hardwood, available in various price ranges and finishes. Most hardwood floors have brown hues. Another alternative to pressure-treated wood is Thermowood, which contains no toxins and is heat-treated pine with a durability comparable to pressure-treated wood. However, it is usually more expensive.
The wood flooring should be installed on battens/rafters in the same material as the planks. The battens must be raised above the substrate to avoid moisture. This can be achieved with tiles or special concrete or plastic blocks. Concrete absorbs moisture as well, and you should therefore place a piece of roofing felt between the concrete and the battens; otherwise, the battens can easily rot before the floorboards.
Fixed beds are the simplest solution for the greenhouse. Traditionally, the greenhouse is set-up with a central aisle with beds on either side, but in the end, it is up to your imagination to create pathways that allow optimal access to the beds and your plants.
If you create a central aisle, you can lower the aisle to create a frost-free cellar. Place one or two foundation blocks on top of each other along the entire length of the aisle and place boards or tiles on top of the foundation blocks.
There are very different opinions on whether a greenhouse floor should be insulated. In general, insulating the floor does not make a lot of sense. During winter, the soil emits heat, and even though it is not a lot of heat, it is better that it can find its way up to the greenhouse, instead of keeping it out with an insulating layer. Some even recommend placing a cement pipe vertically in each corner of the greenhouse, allowing heat to be drawn from the ground.
Solid Greenhouse Flooring VS. Fixed Beds
Solid Greenhouse Flooring
- Stone flooring absorbs the sun's heat, evening out temperature fluctuations between day and night.
- It is possible to cultivate in capillary boxes and pots and having the option of garden furniture at the same time.
- It is easier to rearrange the set-up of your greenhouse.
- It is easier to replace the soil.
- Pots require more frequent watering, and overall, water consumption is higher when cultivating in any container.
Fixed Beds in the Greenhouse
- Better growth conditions for the plants, especially for plants with deep roots.
- Less water consumption and less use of fertiliser required.
- Fewer plant diseases.
- Possibility to create a hotbed with horse manure.
- Slightly more challenging soil replacement.
- Less options for adding garden furniture to the greenhouse as an outdoor space.
- Sick or infested plants in fixed beds are not as easy to isolate from other plants. Plants growing in pots can easily be removed from the greenhouse.
Om Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.
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