April 19, 2018

2015 Low Tunnel Installation

By mid-May 2015, the snows have melted in south central MN, and installation season begins! This growing season, as in seasons past, features two major research sites- St Paul, MN and Morris, MN- in addition to several farmer collaborator sites. This post focuses on the low tunnel installation at the St Paul site, which took place on the third week of May. The soils were well thawed before that time, but rainfall events forced us to wait to install; the bed shaper and plastic layer tractor attachments require dry soils to work properly, and the clay soils on the St Paul site take a few days to drain.

It was a beautiful sunny morning on installation day, and while we waited for the bed shaper to arrive, the well rotovated soil was marked with flags indicating where each row would be shaped. Row centers were spaced 6′ apart, and rows at the St Paul site were 85′ long.

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Rotovated soil with flags marking row centers.

After flags were marked and our equipment arrived, raised beds were shaped in the soil. After shaping, beds are near 1′ above the soil line and 4′ wide. While the tractor typically needs only one pass to make a perfectly flat bed, at the St Paul site two passes were needed. We think this is also due to the clay content in the soil.

After beds are shaped, the bed shaper is removed from the tractor and the plastic layer is attached. The plastic layer accomplishes many tasks at once. Plastic is laid and pressed onto the bed with rollers and wheels while irrigation drip tape gets laid underneath. The burial depth of drip tape is adjustable, and can even be laid flat on top of the bed without burial. We use 4′ wide, 1 mm thick white on black poly plastic mulch for this project. There are discs on the outside of the plastic layer that throws soil onto either side of the plastic, serving to bury the edges. Finally the plastic layer also has spiked wheels that serve to puncture the plastic at the exact spacing you intend to plant. For us, this was double rows with 12″ within row and 14″ between row spacing. When properly calibrated, this device lays and buries plastic mulch and drip tape onto each bed, while also puncturing plastic where you intend to plant, all in one pass.

Plastic layer attachment.

Plastic layer attachment.

Steve Poppe adjusting the plastic Layer. Note the black spiked wheels and white drip tape wheels.

Steve Poppe adjusting the plastic Layer. Note the black spiked wheels and white drip tape wheels.

Raised bed with plastic laid on top and holes punched for strawberry planting.

Raised bed with plastic laid on top and holes punched for strawberry planting.

After the raised beds are completed, planting of the strawberries can finally begin! This year we are planting just one cultivar, ‘Albion’. As we are focusing on nitrogen fertility rates instead of cultivar performance, only one cultivar was chosen to ease the statistical burden on researchers. Plants were acquired as chilled, bare rooted cuttings from Nourse Farms (South Deerfield, MA). Planting is easy- the bottom third of roots are cut to stimulate new root growth after planting, roots are slightly twisted and folded under a wooden stake, and the stake is pressed into each hole in the plastic, bringing the strawberry plant with it. Care is taken that the base of the crown of each strawberry plant is at soil level; if the crown is buried too deep or above the soil line, it can impede growth or even kill the plant.

Strawberry plants being inserted to the holes on each raised bed using wooden stakes.

Strawberry plants being inserted to the holes on each raised bed using wooden stakes.

Earlier pictures show that the furrows in between the raised beds are very clumpy, once again due to the stubborn clay nature of our soil at the St Paul site. After each furrow was smoothed out with rakes, landscape fabric was laid and stapled in the furrows to impede weed growth and allow a smooth walking surface for harvest.

Landscape fabric being laid between rows.

Landscape fabric being laid between rows.

After the plastic and landscape fabric is laid, the time comes for installing low tunnels. As mentioned in the last post, this year we decided to install commercial low tunnels by Dubois Agrinovation. The installation process was a breeze compared to previous years. Stainless steel hoops are spaced 5′ apart along the length of each row, and every other hoop is fastened tight to the ground with thin steel anchors. On each end of the row, thick anchors are hammered into the soil to hold the plastic tunnel coverings.

Daniel installing steel hoops.

Daniel installing steel hoops.

After the hoops are in place, the clear tunnel covering (1 mm plastic poly, acquired from Dubois) is wrapped around one of the thick anchors and then rolled across the tops of each steel hoop in the row. The plastic is cut and tied around the other thick anchor, and then fastened onto the hoops with bungee cords.

Low tunnel covering fastened onto steel hoops. Notice the bungee cords holding the plastic tight on each hoop.

Low tunnel covering fastened onto steel hoops. Notice the bungee cords holding the plastic tight on each hoop.

With 5-6 people working, the plastic covering was able to be laid on top of each 85′ row, taking 10-12 minutes per row. This is a drastic improvement over the installation time of the low tunnels from our previous two years. The low tunnels have held up well so far, withstanding several thunderstorms and some heavy wind events.

The completed site!

The completed site!

The only problem observed so far is that the bungees do not always hold the tunnel coverings tightly to the steel hoops, resulting in the plastic slacking all the way to the soil line. This may prove to be dangerous in late July through August, as we imagine high temperatures can turn the area under an unventilated, unraised low tunnel into a sort of ‘hot box’.  To adjust, we have been rolling excess plastic covering inwards instead of merely pushing it up. This creates more tension against the bungee and seems to do a better job holding the tunnel covering in a raised position.

Plastic covering slacking to the ground.

Plastic covering slacking to the ground.

Excess plastic covering rolled inwards, creating more tension and keeping the tunnel raised.

Excess plastic covering rolled inwards, creating more tension and keeping the tunnel raised.

All in all, 10 rows were prepped for this site project: 5 with low tunnels and 5 without. Upcoming posts will explore the purpose of our project this year, to analyze optimal nitrogen fertility rates for the strawberry crop.