Low Tunnels

by: Dean Gunderson

One of the most adaptable items in the gardens is a good set of wire or pvc hoops to create low tunnels.  Depending on what fabric goes over your low tunnel hoops it can protect crops from early or late frosts, overwinter cold hardy crops, shade crops to keep them cooler in summer and fall, minimize pests, or minimize disease. 

So what is a low tunnel?  Low tunnels, also called quick hoops, are like a small greenhouse that covers a raised bed or rows of in ground plantings that you can’t walk into, unlike a greenhouse or high tunnel.  The structure of the low tunnel that holds the fabric up is usually made of wire or pvc pipes and are bent into an arch shape and just pushed into the ground at the edges of the raised bed or on the edges of the in ground rows of plants.  These hoops hold the fabric material up above the plants so the fabric isn’t just laying on the leaves of the plants.  The fabric it pulled taught and secured to the ground to hold it in place.

How to Build a Low Tunnel

The first step of building a low tunnel is putting the hoops in.  You can use any material that can be made into an arch but the most popular is a heavy wire, which you can bend into a hoop or can be purchased prebent, like these.  Another readily accessible material that can easily be bent into an arch is a thin pvc pipe.  You just push the ends of the hoops into the ground at least 4”-6”.  You should place one hoop on each end of the raised bed or planting row and then another hoop every 3-4’ between those end hoops.  

Then cut your cover fabric of choice 4’ longer than the bed or row you are trying to cover and drape it over the hoops (different fabric options and the uses for each type will be discussed later). 

Then secure the ends of the fabric to the ground.  This can be done in several ways.  The cheapest and simplest is just to put rocks or bricks all along the ends of the fabric to weight it down to the ground.  You can also secure the fabric with sod staples, like these.  Just bundle up the ends of your fabric and push the staple through the bundled edge and all the way down into the soil.  Space these staples every 9”-12” along the sides that don’t need to be opened regularly.  Knowing if it needs to be opened regularly brings us to management considerations.

One of the most important things to remember is that you need to access the plants under the low tunnel.  So you can put the sod staples all the way around the low tunnel but it might be easier to do sod staples on both short sides and one long side but use rocks on the other long side so it’s easier to open and close that size so you have easy access to your plants.  Another thing to remember is that if you are covering the plants to protect from frost for season extension in spring and/or fall remember that our weather can swing wildly in St. Louis in those seasons so having the cover on when it’s cold is important but if then there is a day that is a little warmer, even if it’s not hot, but it’s sunny the plants can actually overheat just like a car sitting in the sun.  So especially those times of years you might need to vent your low tunnel.  This would mean opening up an end of the tunnel during the morning and then closing it again in the evening if it will be cold that night.

The Fabric Options

There are several different options of material to cover the low tunnel with and, which material to use depends on what the goals are.  Below are 4 different cover options for a low tunnel.  Each section describes the material, what it can be used for, and how to manage a low tunnel for that use.

1. Insect Netting: This mesh material is great for covering crops if you are trying to keep pests away from your crops, especially in the summer.  It is airy enough that it doesn’t cast shade and also doesn’t retain heat.  It is also airy enough that rain will go through it so you don’t need to open the low tunnel to vent heat or to water your plants with insect netting.  This can be especially helpful to set up and put over brassica crops to keep out cabbage worms and cabbage loopers and to put over squash to keep out squash vine borers.  It can really be used over any plant to keep out pests and the diseases those pests might be carrying.  The main thing to keep in mind with using insect netting is that a net that keeps out pest insects will also keep out pollinators.  So if you need pollinators in order for your crop to produce, like with squash, you will need to remove the netting once the plant starts flowering or you will need to hand pollinate.  An important note about insect netting is that it is best to cover your plants as soon as you plant them.  If you wait there is the possibility that the insects will already have laid eggs in the soil or on the plant and then if you put the barrier over you are trapping the pest in with your plant.
2. Low Tunnel Plastic: This plastic is simply sheet plastic that can be put over low tunnel hoops for season extension and/or overwintering.  This type of plastic is perfect for putting on in early spring to warm up the soil faster in order to plant sooner or putting on in late fall to keep the soil warm longer in order to continue harvesting later in the year.  If put up in late fall over cold hardy crops it can help to overwinter those crops so they can be harvested all winter long.  Clear plastic will heat up more than white plastic but both are used.  Plastic is the material that will heat up the most so is the best option if you are tying to overwinter crops.  Since plastic doesn’t allow air flow it will definitely need to be vented on sunny days to prevent overheating especially in the spring and fall if used for season extension.  There are also types of low tunnel plastic that have either slits or holes punched in it to prevent overheating, like this low tunnel plastic.  These types don’t need to be vented manually.  Plastic also doesn’t allow rain to go through it so you will periodically need to open it up to water the plants.
3. Shade Cloth: This material looks like a loose weave mesh and is usually made of a colored plastic material, oftentimes black.  It is designed to cast shade on the crops underneath it.  Shade is particularly helpful in late spring-early summer when trying to keep your spring crops cool so they are less liable to bolt and will keep producing as it starts getting hot.  It can also be put on in late summer in order to keep your fall crops cool as the seeds germinate and start growing in the hot sun of summer.  As this is just to cast shade it doesn’t need to go all the way to the ground and leaving some gaps at the bottom helps increase airflow.  As it is a loose mesh rain will go through to water the crops.
4. Row Cover: This is maybe the most adaptable cover for a low tunnel.  It can be used for all 3 of the things the above fabrics can be used for but generally not as good as the fabric that is specially for those 3 things.  It is a spun fabric that looks similar to the type of fabric that dryer sheets are made of.  It traps heat and therefore is often used for season extension.  For protecting from early or late frosts it is generally easier to used than plastic because is less liable to overheat but it is not as good for overwintering because it doesn’t trap as much heat and allows more airflow increasing cold winds.  It is also nicer than plastic in regards to watering because it allows rain to get through which plastic doesn’t.  Row cover also casts shade but only about 15% whereas most shade cloth for vegetables is 30%-40% shade so if doubled or tripled up it could serve as this purpose a little better.  However, as it does trap heat unless you can allow a lot of airflow it can actually be counterproductive if you are casting shade to try and keep things cool.  It is also good as an insect barrier.  In this regard it does just as good of a job as insect netting with all of the same considerations discussed above when it comes to insect netting.  The main problem with using row cover to keep insects out is that as mentioned it also retains heat so can cause overheating when trying to cover squash, brassicas, or any other crop in the middle of summer, when there are so many pests.

Another thing to consider when it comes to selecting and using your low tunnel fabric is that you don’t necessarily have to just do one layer or even just one type of fabric.  As mentioned you could double or triple up row cover to make it cast more shade.  It is also common to do multiple layers of row cover in order to increase how much insulation it provides to protect crops from lower temperatures.  A similar thing can be done with plastic.  Doing two layers of plastic will protect the crops they cover from lower temperatures than a single layer of plastic.  This works because the air between the two layers of plastic acts like an insulation layer.  If you are trying to protect your brassicas from cabbage worms and loopers and also want to cast some shade to help them through the heat of summer you could put on a layer of insect netting and then a layer of shade cloth on top.  

All in all the next time you have an issue in the garden don’t overlook how useful a low tunnel can be and maybe consider using one to increase your yields and lower your work load.