Did you start a late autumn garden, or are your brassicas growing at a glacial pace? Add weeks of additional frost-free growing conditions to your garden with this simple project.
Floating row covers are great, and they’re easy to install, but urban gardeners who use raised beds can create a mini-greenhouse in the same amount of time for the same expense. Even better, urban gardeners who use raised beds are halfway there: the raised bed serves as a sturdy, grounded framework for a simple, quick mini-greenhouse.
You only need two things to start constructing a custom mini greenhouse:
• 1″ Irrigation Tubing. A 100′ section costs about $25.
• 6 mil drop-cloth. Small 10′ length packages cost about $5. We had a bear of a time finding a small package of 6 mil; large rolls are about $25. We found some 4mil for $6 and went with that. (UV-vulnerable plastic only lasts one season, so expect to toss this at Christmas.)
This material is enough for several mini-greenhouses depending on the bed size.
Before you break out the scissors, calculate the length of each support, and its shape. Large greenhouses are half-circles. That isn’t the best shape for small greenhouses. Instead, use a ‘vertical ellipse’ to get some extra overhead in the greenhouse space. You need the extra height because a bed-based greenhouse is so close to the ground.
You only need two numbers to find the best shape: the length of the ‘minor axis’, and an ‘eccentricity’ value. The latter is your choice. The greenhouse you see in this article has an eccentricity of .5.
1. Measure the inside width of your bed. The interior width of the bed shown in this article was 46.5″. This interior width of your bed will become the ‘minor axis’ of an ellipse.
2. Go to this helpful website and enter that number as the minor axis: www.cleavebooks.co.uk/scol/callipse.htm
3. The only other value you need is the eccentricity. Enter a value between 0 and 1. Again, the greenhouse shown in this article has an eccentricity of .5. If your bed is narrower than 45″, consider using an eccentricity of .6 or .7.
4. Click on/tap ‘Calculate It’
5. Write down the ‘perimeter’ value (last item on the page).
Nice work. You just need half that value, and then need to add twice the bed height. Here’s the simple formula:
(Perimeter value/2) + (bed height x 2) = length of each support.
6. Calculate the length of each support piece.
Example: The bed in this article had a perimeter value of 158″. The bed height was 7.5″. So the length of each support came to:
(158/2) + (7.5×2) = 79 + 15 = 94″
The four supports that make up the structure of this article’s small greenhouse were each 94″ long. We cut the ends at a 45 degree angle to create sharp spikes.
Press the supports into the bed evenly spaced, then add a support at the top to strengthen the structure against heavy winds and snow. We just found a scrap, drilled some holes, and zip-tied the mess together.
Drill two holes in the pipe supports and one in the scrap, then snake the zip tie through everything.
Cut the plastic to length, and drape it over the sides and front/back. Pour on some bricks and you’re finished. Now you can weather the colder moments in October and November with ease.
Just remember to open up the entire greenhouse if the weather looks warm and sunny. Cook the plants for dinner, not in your new greenhouse.
