If you are new to season extension structures, and even if you are a seasoned pro, it is hard to talk to three people in the same room that all agree on what makes high tunnels and greenhouses different types of structures. If you are someone who has committed to your own definition of each structure you will probably think that statement is ridiculous, however, in recent years the lines between the two structures have often blended making it more difficult to separate their definitions, and because of that it becomes increasingly difficult for the structure selection process. For these reasons, we are going to provide a comprehensive rundown of these two types of similar yet different structures. More specifically, we are going to cover the following in this post:
- We are going to provide a solid framework for defining both a greenhouse and a high tunnel
- We are going to discuss how to approach picking one structure over the other if shopping for just one
- We are going to cover what more people are doing nowadays: making a structure that acts as both a greenhouse AND a high tunnel
- We are going to tackle potential permitting considerations when selecting either type of structure
Defining a High Tunnel and Defining a Greenhouse
To compare a high tunnel and a greenhouse you must first define a high tunnel and a greenhouse. Defining each sounds like an easy enough task, however, the lines between these two growing structures have often blurred, especially as small farmers have done their best to squeeze everything possible from their structure(s). That said, it is still important to set the parameters by which you want to define the structures so you can better compare them and decide which is right for you. With this in mind, there are two primary ways you can approach defining a greenhouse or a high tunnel:
1) You can define either structure by how they are being used
2) You can define either structure by its structural features and architecture
Which of the above ways to classify a structure is correct? That can be up for debate, but if you read on we feel we've made some compelling reasons why it may make sense for you to set some firm boundaries based on architectural features, especially when your money or time could potentially be on the line if there are ever issues with your municipality regarding permitting, usage, occupancy, and more.
Below is how one would approach defining each structure by usage or features.
Defining by Usage
If you were to define a greenhouse or a high tunnel hoop house by what their use the conversation would likely be much easier. For example, a traditional greenhouse is often used for seed propagation, potting and re-potting plants, and commercial retail sales. Alternatively, one could define a high tunnel or hoop house as a structure where in-ground planting is the primary activity. Many people reading this will read this, and think that the above is cut and dry, but high tunnels have increasingly been used for seed propagation, and potting in addition to in-ground planting.
Defining by Structural Features
Here is how we would take on defining each structure if going by structural features.
Traditionally, greenhouses include concrete footers as anchors, or more often than not, a complete concrete pad. Additionally, because greenhouses are being used where absolute environmental controls are often required, these structures tend to also include complete utility hook-ups in the form of water, gas, and electricity. Greenhouses are are also more likely to include hard greenhouse plastics such as polycarbonate sheeting.
From that same traditional point of view, a high tunnel hoop house would not be anchored with concrete footers, would not include heaters or permanent utility hook-ups, and would be covered by a low cost plastic greenhouse glazing.
Again, lines are blurring all of the time with both greenhouses and high tunnels, and while it is likely a good thing for growers to blur these lines, the differences in the operational definition of a greenhouse among farmers, and even among those in the greenhouse industry sometimes make pin pointing the type of structure that is right for your farm even more difficult...for example, it may be difficult to receive an accurate quote on a structure if everyone has a different definition of a greenhouse or a high tunnel.
The Real Defining Structural Features
We are just going to set our own parameters for the purpose of this post. Below we provide our best definitions of both a high tunnel and a greenhouse.
For the purposes of this conversation, a high tunnel is a plastic covered growing structure that doesn't have a concrete pad, and doesn't have permanent utility hook ups. In its truest form a high tunnel wouldn't include concrete footers either (different than a concrete pad), and would instead include a non-concrete anchoring method. In short, a High Tunnel boils down to a poly covered structure where in-ground planting is the primary purpose.
As for a Greenhouse, we define a greenhouse as a permanently anchored structure that includes permanent utility hook ups such as gas, electric, and water. If you wanted to take this a little further you could say that greenhouses are also more often covered with a hard glazing in the form of twin wall polycarbonate, or glass. Additionally, you could take it even one step further than that, and say that greenhouses often include permanent or semi-permanent growing platforms throughout their interior that make seed starting more ergonomic (greenhouse benches). From a structural perspective, the foundation, drainage capabilities, and utility set up is more defining of the greenhouse than anything else. For example, if you have a structure with a concrete pad, and it has a 100,000 BTU heater, and full water and drainage capabilities, but it is covered with a simple 4 year 6 mil Clear greenhouse plastic, as opposed to polycarbonate is it a greenhouse? Of course it is; just because it's covered in a ply-able plastic doesn't mean it is a High Tunnel. It's foundation and utility capabilities put it firmly in the greenhouse category.
So, which one is best for me?
As mentioned above, some of the defining qualities of a high tunnel and greenhouse can overlap. For us though, if the intent of the structure is to focus 100% on seed starting and you are going to need to keep those seedlings warm in the beginning of the season, perhaps a greenhouse would be a more appropriate structure.
If the intent of the structure is to focus on in-ground plantings that are planted based on seasonally appropriate crops (such as tomatoes in the summer and carrots or kale in the colder months) than a high tunnel is probably the structure you need.
Can I do Both in One?
If you have read the above and thought to yourself, “it isn't as simple as that; I want a structure that can act as both a high tunnel and a greenhouse” than you aren't alone.
Some high tunnel operators do opt to add a heater for their structure just to ensure the interior temperature doesn't dip below 32 degrees F in the winter months. This allows their cold loving, in-ground planted crops to thrive due to the fact that the ambient air temperature never dips below that magical 32 degrees. Sure, many crops can withstand temperatures even lower than that (Asian greens, spinach, kale, etc.), but having a bit of supplemental heat often allows those plants to experience less shock than if there was otherwise no heater included in the set up. This structure configuration would still be considered a High Tunnel although its inclusion of a heater starts to make the lines blur again. The fact that it doesn’t include concrete footers or a pad though make it more just a “high tunnel that had a heater installed in it”.
Be careful when taking the combination route though, as many municipalities may take issue with this logic with regards to permitting, so it is important to check in with your local municipality before deciding how to configure your structure.
Half Greenhouse Half High Tunnel
More often people are starting with a high tunnel kit, and working with manufacturers to develop a structure where part of the square footage is dedicated to in ground planting, and the other part of the structure is dedicated to working as a greenhouse. This often includes interior partitions that vary in their level of complexity, as well as the addition of a heating element. Some even add a concrete pad and water drainage capabilities for their greenhouse portion of the structure. There are pros and cons to separating a structure like this, but it is happening more and more, as smaller farms don't grow on enough land to justify the purchase of two separate structures.
Whether you've set up a structure to be a high tunnel hoop house, or a greenhouse, or a combination of both, the way the structure has been configured could have large implications as far as permitting goes. Permits are issued based on different guidelines depending on what municipality you are building your structure in, and the hoops you need to jump through (pun intended) to get a permit differ widely.
Some states have what is called an agricultural exemption whereby structures being built for the use of agriculture have no need to get a permit. This agricultural exemption would / should cover the construction of high tunnels or greenhouses without a permit being sought, but some municipalities will still make you get a permit in there municipality despite the statewide language of the agricultural exemption. Whether the local municipality can do this legally is out of my expertise, however, if you want to build your structure without any issues it is best to try and work with the local municipality even if your state government is saying something different.
Is it easier to gain a permit for a High Tunnel or a Greenhouse?
If we are comparing high tunnels to greenhouses with regards to which one needs permits, and which one may be able to get by without a permit high tunnels are the clear winner.
High tunnels, in addition to being agricultural structures, can often qualify as temporary use structures due to the fact that they won't be anchored with concrete footers, and they won't have a concrete pad. Municipalities may also require that heating elements not be used as well since those would require permanent utility hook ups.
For high tunnels, the fact that it might be considered a temporary use structure by the municipality will help the permitting process move much more quickly in some instances. High tunnels are also less likely to require the need for engineer stamped drawings to accompany a permit application, which is a big deal due to the fact that engineer stamped greenhouse drawings are usually required on a per project basis, and usually add moderate to significant expense to the purchase of a structure.
Approaching a Municipality...or Not?
Some people ask whether they should approach their municipality about a permit or not. While some people swear it is better to ask for forgiveness rather than permission, we have heard enough horror stories to recommend approaching your municipality before proceeding with a structure purchase or build. It is advisable to consult your local building department to see how you can navigate building a high tunnel or a greenhouse. Since many municipalities may be unfamiliar with the structures it would be wise to try and build a portfolio of examples that could be shared with your contact at the municipality that shows the structures, and the anchoring methods for the structures. In our experience, this approach has helped customers avoid future headaches, and in some instances municipalities have allowed structure installations to proceed without the seeking of a permit at all.
Setback requirements, the number of structures allowed on a parcel, and other guidelines should be followed whether a permit is required or not, as it will prevent potential future problems should any changes in policy take place. Again, working with your building department is strongly advised, and if permit restrictions are one of the main concerns you have when selecting to build a greenhouse or high tunnel it may be wise to pursue a high tunnel due to the less stringent (often) permitting process that accompanies these types of structures.
I need both a greenhouse and a high tunnel but only have money for one, which should I prioritize?
This is a good question, as it is likely a question many small farmers will have to ask themselves as their business grows.
There is no cut and dry answer other than to say the answer to this question will depend completely on the farms business model. For example, if a farm is focused on a CSA program, and having a high tunnel will allow them to extend their CSA by 12 weeks into the fall and early winter, perhaps it makes more sense for that farm to prioritize a high total purchase so that they can grow their revenue and income stream not only for that year, but for all CSA seasons to come. In that scenario the extra income could go toward a future purchase of another structure that's more focused as a greenhouse growing structure.
In a different scenario, perhaps a farm is looking to expand into a different revenue source by spreading into the selling of plant starts. Starting plants from seed, and selling off the young plants has become a growing supplemental farm revenue source for many farms, and there are many farms that completely focus on plant starts as their sole and primary business. This would obviously require seed starting to be a primary use of whatever structure being purchased, and for this scenario a greenhouse would seem to be an ideal growing structure. In an operation focused on starting seeds, a greenhouse would not only provide benefit in its smooth floor surface, but it would provide value to the grower if outfitted with a heating unit; an operation focusing on plant starts should strongly consider a heating unit, even if only as an insurance policy against the unpredictable weather outside.
Hopefully this post has helped draw some lines between high tunnel hoop houses and greenhouses, even while many of the lines between them blur. If you are interested in working through your own deliberations with one of our tunnel specialists please feel free to shoot us an email at firstname.lastname@example.org or give us a call at (833) 886-6351.