What's the difference between a caterpillar tunnel and a high tunnel?
The difference between a caterpillar tunnel and a high tunnel comes down to three things- permanence, structural soundness, and budget. Both structures have their place in the gardening world, but they have sharp differences that need to be accounted for by the grower.
Using covered growing spaces for season extension is a time honored, low cost practice that extends the growing season for farmers and gardeners in almost any growing zone. While greenhouses are often expensive infrastructure and cold frames are too small for market gardeners, tunnels are a happy medium that offer multiple opportunities.
Fall planted cool season annuals for early flower production
Protected early Spring greens for first to market possibilities
Extended harvests in the heat of the Summer using shade cloth
Added degrees of protection in late Fall for specialty holiday meal crops
The caterpillar tunnel received its name because of its close resemblance to a caterpillar. It has segmented sections and cone-shaped ends that hug the ground at either end, usually held together by a stake and rope. The plastic is secured down by side-to-side crisscrossing nylon rope anchored to the base of each pole (where they touch the ground). It does not have built-in endwalls and is considered a temporary/movable option, allowing it to be one of the cheapest and quickest options. Many market gardeners use it as a starting point, often than later converting the caterpillar tunnel into a high tunnel greenhouse.
A high tunnel or hoop house, on the other hand, does have endwalls, doors, and sidewalls. Instead of a rope system, the structure is secured by metal base and hip board, side-braces, and purlins. The hoop house is then anchored to the ground by heavy-duty metal ground posts. The plastic is secured by metal lock channel and PVC spring wire, giving more stability and reliability. With the installation of hand cranks, the plastic sidewalls are often made to roll up and down, allowing for easy ventilation. This type of infrastructure is an investment meant to last for many years as they are a more durable, long-lasting option. Read Prequel to Building Your First Hoop House to learn more about how to build hoop houses.
Greenhouse coverings for use with caterpillar tunnels and high tunnels.
There are a variety of options when it comes to covering your hoop building. The four most popular greenhouse coverings are:
A basic caterpillar tunnel can usually be built for around $1.00 per square foot. This reflects the recent dramatic increases in raw materials costs that we have seen across industry since the pandemic has caused shortages and shipping delays. In the recent past it was possible to build one of these for closer to $0.55 per square foot and we are hopeful that prices could return to this level in the next year or two. Their low cost is one of the reasons caterpillar tunnels are often a grower’s first foray into protected growing space.
How much do high tunnels cost?
A DIY hoop house kit can be an affordable starting point for many farms for season extension. A basic kit can be purchased for under $500 to be built with materials from your local hardware store. If you are looking for a more heavy-duty all-metal hoop house, you can buy all inclusive kits like the Bootstrap Farmer All-metal Greenhouse kits, without any need to get supplies from your local big box store.
Due to the reliability of these structures the USDA has put into place a grant program to help alleviate the investment costs of adding hoop houses to a farm’s operation. This program is called the NRCS High Tunnel Initiativeand qualifying hoop houses are funded by this program for many growers around the country. Because this piece of infrastructure can sustainably extend the growing season for many farmers it helps increase food security. Because it improves and protects soil conditions it is supported by funds set aside for soil conservation.
How does a caterpillar tunnel work?
Cat tunnels are deployed with the help of a rope system, often anchored by a t-post or sand bags. Instead of a purlin system, this tension rope technique offers the structure its form at a lower cost and a more mobile design. This is why this style of growing structure is popular amongst startups and with those not needing heavier duty infrastructure. The main downside is that without the structural soundness of a purlin kit or ground posts, the plants you are protecting could be damaged by impaling winds and weather if wind were to grab the insecure structure. Taking care to select the right locationfor your structure, away from obstructions and protected from winds will also help mitigate this risk. Read Ideal Orientation and Location to learn more about what direction to face your hoop house or greenhouse.
What do you need to build a caterpillar tunnel?
You will need pipes to build your hoops, either EMT(Electrical Metallic Tubing) or PVC. Rebar to use as ground anchors for your hoops. Rope or strapping to attach the hoops to each other and to secure your greenhouse covering to the hoops. You can also use special greenhouse clipsdesigned for PVC or EMT to attach your greenhouse covering.
Since caterpillar tunnels are essentially a low hoop house without doors, the ends of your tunnel require little more than a smooth rock and some rope to gather the ends of the plastic covering and weigh it down.
Hoop building for a homemade caterpillar tunnel
Hoops built with PVC are typically just slid over rebar anchors at both ends to make the hoop shape. It is best to do this part on a warm sunny day as the heat makes the pipes more malleable. This malleability is also what makes tunnels made from PVC weaker overall hoop structures.
Caterpillar tunnels in their simplest form are structures used for season extension as a temporary covered growing space for crops in production. They are often used to warm soil temperatures to prepare for spring planting. Flower farmers plant their early bulbs under caterpillar tunnels to get a jump start in order to bring their flowers to market sooner. Vegetable growers take advantage of the cover and warmer soil temps both early and late in the season for their production.
Tunnels covered with frost blanketare often used to continue growing frost hardy plants through the Winter months. Vegetable crops like kale and broccoli that will be harvestable in the cold can be protected from snow in this way so that you can access them for harvest. For flower farmers, planting cool season annuals like snapdragons and bachelor buttons in the Fall and covering them with low tunnels allows the plants to develop roots through the Winter and be ready to take off as soon as the weather warms.
Caterpillar tunnels covered with shade cloth are used in the hotter months to protect tender crops from excess heat. They are also used in this way to protect fruiting crops like tomatoes and cucumbers from sunscald.
How to anchor a caterpillar tunnel
Caterpillar tunnels can be anchored with rebar, ground posts, sandbags or with a rope system secured with a t-post. This method will be dependent on the permanence you desire from the structure. One of the advantages a caterpillar tunnel offers is its easy disassembly. This allows farmers to use the structure temporarily in one place with the ability to move it when no longer its need is no longer warranted. If the grower prefers to make their caterpillar tunnel more of a permanent structure they can add rebar or ground posts to create more of a hybrid caterpillar hoop house structure.
How to Reduce the Chance of Wind Damage to a Caterpillar Tunnel
A great way to reduce wind damage to a caterpillar tunnel is to securely attach the plastic. You may want to consider adding base and hip boards to your structure. This will eliminate the danger of the wind lifting the structure. You may also want to consider installing ground poststo the structure. Caterpillar tunnels are often vulnerable to wind because of the “parachute” effect that occurs when wind gets under the structure so it is important to have it anchored to the ground.
What can I grow in a caterpillar tunnel?
Caterpillar tunnels can be used for season extension or to allow you to start your season just a bit earlier. They work well for warming the ground for earlier planting and to protect new plants from late spring frosts. While they offer a great temporary protection, their basic design can succumb to winds and snow leaving your plants vulnerable to the elements if a failure occurs. The simple rope system is sometimes not a match for extreme weather. This is why we only recommend them for season extension and for warming the ground for earlier planting.
What can I grow in a high tunnel?
A high tunnel gives the grower more assurance to grow later into the season. The durability of the metal frame is enough to protect from snow and wind that sometimes comes with the fall season. Vegetable crops and flowers have ample space and protection under a tunnel. Read Top 5 Reasons Why You Should Build a Hoop House Style Greenhouse for more about the benefits of high tunnels.
How to Convert a Caterpillar Tunnel into a High Tunnel.
Caterpillar tunnels converted to high tunnels are referred to as hybrid structures. They combine features of both the caterpillar tunnel and the high tunnel or hoop house. Some farms upgrade their 'cat' tunnels using ground posts, giving them more height along the sides. Common upgrades include reinforcing the center purlin (from strapping or rope to metal pipes) and adding endwall supports. Modifications often reflect a farmer's budget, preference, and growing conditions. While converting your caterpillar is an option, one might consider if the added cost of shipping parts to modify your past purchase makes sense. Putting an investment toward a more permanent structure from the start will give you the features that you want in one firm price instead of slowly adding at retail along the way. For this reason, we suggest looking at your farm's future needs along with current needs when purchasing infrastructure.
Should I buy a Caterpillar Tunnel or a Hoop House?
The decision on whether to buy a caterpillar tunnel or hoop house to create a covered growing space really depends on your farm's needs. While the caterpillar does have a smaller startup cost, the stability and reliability of the structure do not meet that of a high tunnel. Both structures need to be shipped freight and both structures require help to build. If you believe you will need something sturdier in place for the future, it might be better to get it right the first time instead of slowly creating more of an investment with add-ons.
To learn more about how farmers and market gardeners are using high tunnels and other covered growing spaces follow us on Instagramand subscribe to our YouTube channel. We love to feature stories about how our customers are rocking it on the farm.
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