By Jeff Begin
July 3, 2022
In this post, we'll dive deep into row covers: types of covers, their various uses and how to set one up in your garden.
What are low tunnels and row covers?
Low tunnels are common in commercial agricultural production. They can be described as any type of row covering fabric suspended or placed over a crop that is low to the ground but high enough to clear the crop it covers. They usually cover only a single row or bed of crops and are not high enough to walk under. You usually lift them or remove them to access the crop.
But as we'll see, you don't actually need the "tunnel" part to use row covers, you can lay them directly on the plant. So I'll use the term row tunnel throughout to refer to either configuration.
Why use row covers
Row covers have three primary uses:
- Pest exclusion
- Season extension and frost protection
- Shade cloth for growing cool-season crops in summer heat
How you design your row cover system and what row cover fabrics you use will vary depending on your goal. The good news is row covers are not expensive, if you keep it simple.
Row cover configuration
Row covers can come in two general flavors: floating row covers and supported row covers or low tunnels.
A floating row cover is one where the fabric is draped directly over the plant and "floats" on top of the plant. The plants underneath support the cover and push it up as they grow.
The example below is a row cover of bird netting laid floating over squash.
A low tunnel or supported row cover uses hoops of varying material, but typically galvanized wire or PVC tubing to create an arch that suspends the row cover up off the plants.
The example below uses insect netting suspended over galvanized metal wire made into support hoops. The netting is not touching the plants underneath.
Types of row cover fabric
There are several types of row covers that are made to suit different purposes.
The first few types of row covers are "spun" polypropylene fabric. They're not a fabric as in a cloth fabric, but a very light web of polypropylene fibers all stuck together. The manufacturing process allows for creating fabrics of varying weights or thicknesses that allow for different uses throughout the season. In general, heavier weight fabric traps more heat than lighter fabric. The weight of fabric is generally listed in ounces (of fabric) per square yard.
Insulating (medium-heavy weight) spun polypropylene fabric
Fabric marketed as medium weight fabric usually has a weight of between 0.4 oz to 1 oz per square yard. Heavyweight fabric is anything over 1 oz/sq yd.
This fabric is insulating in the sense that it will trap more of the sun's heat in the tunnel. When the sun is shining and depending on the color and material of the surface mulch, the tunnel can be perhaps as much as 7-10ºF warmer than the outside temp. At night, the temperature inside will probably not be significantly higher than the outside temp, often said to be 4ºF of "frost protection".
These fabrics will last about 4-5 seasons, in my experience.
Lightweight/summerweight spun polypropylene fabric
This is any fabric less than 0.4 oz/sq yd, but often much thinner than that. Ideally, this type of fabric will retain little to no heat in direct sunlight, though in practice there will always be some.
I have used this summerweight fabric from Gardeners.com for many seasons and like it a lot. It does not build any real heat (see note below about use with black plastic mulch) and transmits nearly all light. This makes it perfect for pest exclusion.
But the lightweight fabric means it's not very durable. Expect to get between 1-4 seasons use before needing to replace it.
Insect mesh row cover
Insect mesh offers little to no insulation or heat retention, which is usually a good thing in the time of year when you're trying to exclude insects. It is basically a window screen, in white and on a long roll.
The mesh comes with different mesh opening sizes to deal with different insects. In general the smaller the mesh openings the better, though that also means a denser mesh that will build up heat more easily and transmit less light.
The picture above is this Agtec product I ordered from Agricultural Solutions and it's worked well.
This material is durable and should last ten seasons.
Bird netting has a very wide and open mesh that is wide enough to allow insects to pass through easily, but keep out larger animals. The opening is usually about 1/2" square.
See the image for the floating row cover above for an example of this material.
This material is useful when you want to exclude larger pests such as rabbits, woodchucks, deer and of course birds, but have absolutely no heat build up and allow through pollinating insects.
Quality of construction varies widely. The bird netting at the big box stores is so thin it's probably not even going to survive opening the package. On the other hand, I can warmly recommend the bird netting sold by Gardeners.com as it's very durable. I expect it to last at least 10 seasons and probably many more. I only wish they sold it in larger row-length sizes like 6'x100'.
Shade cloth is used to reduce the total sunlight transmission and therefore heat under the cloth. I use this to grow lettuce in the summer heat.
I purchased the above netting from ShadeClothStore.com and am happy with it's construction for the price.
The mesh opening is somewhere between insect netting and bird netting. It is not large enough to allow bees through but will allow smaller insects.
It is important when using shade cloth as a row cover to ensure the shade cloth is not touching the plant anywhere, and ideally is suspended well above the plant. If the black shade cloth touches the plant, it will damage the plant due to the heat of the black strands.
Greenhouse film as a row cover
For the ultimate in season extension, heat preservation and frost protection, you can use greenhouse film as a row cover. This essentially makes your low tunnel into a greenhouse, allowing you to plant late into the season and start early.
Installing your row covers
Adding row covers to your garden does not have to be expensive. The covers themselves tend to last a while and aren't very expensive to begin with.
DIY, scalable and cheaper option
This is the method I use here on the farm. For the covers I use pretty much all kinds for various purposes, but I only buy them in 100 ft lengths, as that's the length of my longest beds. I don't cut the cover to fit shorter rows, I just roll it up.
For the support hoops, I use 9 gauge galvanized steel wire. This wire, to be specific. For 4 ft wide beds, I roll out a 6 ft long piece of wire then cut it with wire/bolt cutters. It should already be in a crescent shape ready to use as a support hoop. From there I just stick it in the ground on either side of the bed, and lay my row cover out over it.
To hold the row cover down I just use rocks from around the property. Being in New Hampshire, those are in plentiful supply.
The Cadillac option
If you want an all-in-one top of the line solution to just get going, I have heard good things about the DuBois Tunnel Flex low tunnel kit, though I haven't used them personally. It runs a little over $500 as of this writing and also comes in 50' lengths. But it has lots of conveniences, such as the ability to fold up the sides and have them stay there. It also looks very sturdy for holding up in high winds, which can occasionally be a problem with my DIY solution.
How to use
So we've been through the various types of row coverings, what they're used for and the options for support hoops.
But how do you actually use them? When is it best to put the covers on and take them off?
Frost protection/season extension
This is a big reason for using row covers in many operations. Warmth in early spring will speed along growth of new transplants, increasing yield later in the season. Use a heavyweight cover or greenhouse film for this.
You can also extend your season later into the fall using the same method.
But a warning - these row covers will build up heat. It's important you remove them on any day that's going to be 70F or higher. If you're using greenhouse film it's probably a lot lower than that, maybe above 55F. If you don't you can kill your plants. If you start getting continual highs in the 70s or above, it's time to switch to a lighter fabric.
I use row covers mainly for this reason. I don't spray my crops and I don't have a fence around my main growing area, so I need to cover the sensitive crops. See my posts about specific pests and when to apply covers in the other pest control posts in this series (links at the end of the post).
With insects and insect mesh, it's important to know when they will arrive, and watch for them. If you put the cover on and trap an insect inside, it can reproduce in the tunnel and before you know it you're trapping insects in the tunnel rather than keeping them out. For that reason it's best to apply the cover before you think you'll need it and remove it as little as possible.
Shade cloth is simple to apply, the only thing to be aware of is ensuring the black shade cloth doesn't touch the plant or it'll burn it. You can't do floating row covers with shade cloth.
Transplant establishment and seed germination
Putting a row cover over tender transplants is a great way to protect them. Young plants are at their most vulnerable when small, as a small nibble may set the plant back the whole season. Young plants are also most attractive to pests.
Seeds are also a prime target for pests, especially birds. Putting bird netting or any other cover over a row with direct-sown seed us a useful insurance policy.
A word about row covers with black plastic film
If you use black plastic film as a mulch, like under tomatoes, be aware that will trap and release more heat than usual. If you have a row cover over a row with black plastic film, be sure your row cover doesn't trap any heat, because there will be a lot of it with the black film.
I suggest only using black film in rows you know won't need a row cover.
Row covers are indispensable on our farm, as they are in most organic operations. Traditional agriculture can make use of pesticides but in organic practices there aren't nearly as many or as effective options for sprays. The best bet is exclusion.
Row covers will make you more successful in your gardening, so give them a try!
Other posts in the pest control series: