Propagation House Essentials
Matt and Jenny during construction of their first propagation house - circa ~2017
A propagation house, also known as a greenhouse or a plant nursery, is a place where you start seeds or propagate plant material. A propagation house was one of the first things I invested in, since I was tired of poor results of starting seeds indoors (and if I’m honest, I couldn’t stand the mess in my house any longer!). You need high-quality flower starts (aka transplants) to grow high-quality cut flowers! A greenhouse was the best option for us to do that.
In this guide, you’ll learn all about the basic essentials needed to consider when investing in a propagation house.
THREATS
Greenhouses are wonderful, invaluable structures that can greatly extend your season and improve profits on your farm. However, there are hazards and cautions you should be aware of when considering building a greenhouse. Here are some of the common threats that we’ll be addressing in this guide:
Cold climates and snow: Snow and ice are major threats to greenhouses. The weight of heavy snow and ice is easily enough to crush or collapse structures. The shape, design, material, and quality of the greenhouse needs to be taken into consideration to minimize risk of a collapse.
Heat: in hot weather, your greenhouse should be equipped to ventilate and/or be cooled down so temperatures don’t reach dangerous heights. The sun can easily heat a greenhouse to 90*F+, even on a 30*F winter day.
Humidity: Your greenhouse will naturally (most of the time) be high in humidity because it traps respiration from the plants inside. You will sometimes need to lower the humidity to avoid fungal diseases, which is commonly done by venting. Your greenhouse should have a system to accomplish this.
STRUCTURE SHAPES
Propagation greenhouse structures comes in different shapes, the two most common structural designs are:
“Gothic” structures: these have a more pointed roofline and peak (similar to A-frame) with straight sidewalls. These are used most generally in areas that get lots of (or any) snow, since the A-frame shape helps shed snow off the roof and helps avoid the structure collapsing.
“Quonset” structures: these have a round roofline with shorter and curved sidewalls. These are most commonly used in areas that don’t get any or much snow. Some quonset structures do just fine in cold climates with lots of snow, it really depends on the shape and angle of the roofline. Snow can always be manually removed, but it can be more difficult with a quonset structure.
STRUCTURE SIZES
When choosing a greenhouse size, you may have to do some math. Consider how many trays you will have started at the busiest time of the year in the nursery. If your trays are 10”x20” (the standard 1020 tray and cell tray size), how many can you fit on your benches? How many benches can you fit in your greenhouse footprint? Be sure to consider building a size that can accommodate future business growth. Our propagation house is 20’x40’, with three benches measuring approximately 4’W x 35-40’L. Doing the math, I can fit about 75 trays per bench, which means I can fit 225 trays at a time in my greenhouse. In a pinch, we can fit more by putting trays on the floor or under benches, but it’s not ideal. Only you can figure out the best size for your operation and what you envision the future to be!
SOURCING, SUPPLIERS & OPTIONS
There are too many greenhouse structure suppliers to count, but you can easily search your local area or online to find companies near you. I don’t recommend getting structure packages shipped to you from far away/other states because shipping can get real expensive. Often, you can find used greenhouse frames on facebook marketplace or craigslist (which is where we got our propagation house frame!).
I will mention Farmers Friend here, because I think they make a great product (no affiliation) and you could potentially use one of their caterpillar tunnel kits to “beef up” into a prop house (plus they keep their shipping rates reasonable). You can also bend your own hoops, if you are handy. Johnny’s selected seeds has some bend-your-own hoops information on their website- however, be wary of using this technique if you want your greenhouse to last a long time.
When shopping around for a greenhouse frame, make sure you buy one that can withstand any crazy weather you may get. High winds or snow will mean you want heavy gauge steel. For example, our propagation house frame is made of steel I-beams, and our high tunnel frames are made with 2-3/8” thick steel to stand up to heavy snow load. Conduit or EMT frames simply wouldn’t hold up well in the long-run with our wind, snow, and general inclement weather in NY. In addition, you will want to secure your structure well with purlins, hip-boards, base-boards, and additional rigid bracing between the end walls and first few hoops on each side.
You’ll need to add heat to your propagation house (more on that later), which means you’ll want as much R-value as possible to hold in that expensive heat. Using two layers of UV-resistant plastic on your greenhouse with an inflation fan is the best and most cost-effective way to accomplish this. The inflation fan simply blows air between the two layers of plastic, creating a “bubble” that helps hold in heat. It also greatly helps snow shed off the structure - a must in heavy snow load areas!
Endwall options: you can add a single layer of plastic to your endwalls, which is the most economic way to build a greenhouse. With this option you get the least amount of R-value. Double plastic is better, and polycarbonate is best.
HEAT / HEATERS
There are many options out there for heat sources: wood, propane, electric, passive solar, geothermal…the list goes on. However, I find that the most efficient, easiest, and most affordable way to heat is with propane. It’s safe, easy to get, and in plentiful supply. It’s historically always been cheaper to buy than other fuels.
Propane heaters are easily found at any home improvement/hardware store or online. Before you purchase one, make sure you are getting one with enough BTU’s to heat your space efficiently. Here are two great articles for figuring this:
How to size a greenhouse heating system
Greenhouse heater size calculator
*Pro-tip: Consider getting two smaller heaters instead of one. In the event that one breaks, you’ll have a built in back-up! You’ll have to consider back up heat options regardless.
Another consideration is using perforated plastic tubing to evenly distribute the heat throughout your greenhouse, ideally below the benches so the heat rises to hit the bottom of your growing trays. This isn’t something we have on our farm, but will probably upgrade someday to include it.
VENTILATION & COOLING
Just as much as you’ll need heat for your greenhouse in winter, you’ll also need ways to cool it down in spring and summer. Roll up sides work great, since you can simply roll up the sides for ventilation and to release heat and humidity as needed. Shade cloth is another good option, but you will have less sunlight entering the greenhouse (plus it takes time and energy to put it up and take it down each season).
Be sure to install an exhaust fan and hook it up to a thermostat so it automatically turns on when temps hit a certain high. An electrician can easily do this for you. Horizontal Airflow Fans (HAF) are also essential for providing enough airflow in a greenhouse. They’re simply small, powerful fans that move the air horizontally throughout your greenhouse. HAF fans prevent stagnant air and prevent disease.
BENCHES
When you buy or build your greenhouse benches, be sure to use either plastic or metal bench tops (if you can swing it). Wood works, but it will absorb moisture and disease, and can’t be disinfected. Rodents and mice also can easily climb up wooden table legs, while metal legs make it harder for them to reach your valuable seedlings. We still have wood bench legs, with hard plastic benchtops in our prop house - we just keep a close eye on rodent control. You can always start with wood and upgrade to metal or plastic later. One key consideration is to make sure your bench tops are level. Uneven bench-tops will result in uneven watering, since water will travel and pool to the lowest parts of the cell trays.
WATER
This one probably seems obvious, but your greenhouse will require daily watering which means you’ll need a steady source of water. A frost-free hydrant with an elevated hose system is life changing if you can swing it! Warm water is best, so it doesn’t shock your plants, but if you can’t make it happen at first that’s OK. You can install a hot water heater, or simply place large black (opaque) water tanks with a water pump in your prop. house. The black tanks will heat up the water during the day, and when they empty you can simply fill them back up again. This also will help keep your greenhouse warmer at night, decreasing the need for propane heat.
I hope this helps with your farm infrastructure upgrades!
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