Capture more light for your plants

Light is one of the key components needed for growing plants. A rule of thumb is “1% increase in light equals 1% increase in plant growth.” How can we increase the light level, especially during the dark days of winter?

Research has shown that greenhouse film and polycarbonate lose 2% or more light transmittance each year because of ultraviolet light. Depending on where your greenhouse is located, dust, dirt and air pollutant accumulation may also cause light reduction. A 5% reduction per year is likely in some areas near large cities on the east coast, Midwest or west coast. Industrial areas also tend to have greater smog (smoky fog) concentrations.

I recently visited Freund’s Farm Market and Bakery in East Canaan, Connecticut, to do an energy audit. Theresa Freund indicated that they had replaced the 20-year-old polycarbonate on their 7,200 sq. ft. gutter-connect greenhouse. She found this resulted in having to rework the plant production schedules as the increased light reduced the plant production time in the spring by two to three weeks.

New glazing can reduCe the heating cost

A simple calculation can be made to estimate the fuel savings for the reduced time when the greenhouse is not heated. Determine the heating degree days for the year. This can be calculated from degree day data available for any location in the world on the website www.degreedays.net. Based on research at Rutgers University, select a temperature that is 50 less than the night operating temperature and let the program calculate the number of degree days. Solar energy makes up the 50 difference. Divide the estimated gallons of oil or propane or ccf of natural gas used for that greenhouse for the year by the number of degree days to get the fuel use/degree day. From the degree day calculations, determine the number of degree days during the period when heat is not needed and multiply by the fuel use/degree day. Then multiply by the cost of the fuel to get the savings.

The Freund’s Farm greenhouse used 3,880 gallons of propane for 1,400 degree days for the year 2020, or 2.77 gallons/degree day. There were 400 degree days in the first two weeks in March when the greenhouse didn’t have to be heated, resulting in 1,108 gallons being saved. At $3/gallon propane, the savings for not having to provide heat resulted in a savings of about $3,324. This helps to offset the cost of the new glazing. There is also a savings in labor not needed to maintain the plants for two weeks.

For air inflated greenhouses, replacing the plastic yearly may be economical. During the summer, light levels are high enough that the reduction in light may not affect plant growth. It is during the late fall, winter and early spring when this is of most concern. Let’s look at an example. The cost of replacing the double layer of plastic on a 30’ x 100’ hoop house is about $1,500 ($0.50/sq ft for plastic and labor x 3,000 sq ft = $1,500). It would take an increase in production of just 0.63 lb/plant of tomatoes at a wholesale price of $3/lb to pay for the new plastic. Increased yields of one pound/plant or more are not uncommon when new plastic is installed.

Now that we have taken care of the easy upgrade to the greenhouse, let’s look at some other areas where light getting to the plants could be increased.

Inside the greenhouse

• Place water, heat pipes and conduits below ground, along the post line under the gutter or along the sidewalls.
• Install energy/shade screens that retract into a small space. When retracted, some shade systems create more than 10% shade.
• Locate overhead heaters near the north wall of the greenhouse when possible.
• Use a glazed louver system rather than motorized metal shutters.
• Remove insect screening and evaporative pads during the winter.
• Remove overhead hanging basket supports for the winter. Eight rows of 1-1/4” support pipe create more than 5% shade in a 21’ bay.
• Paint structural members, bench surfaces and walkways white to reflect light. White paint will give about 80% reflection, whereas aluminum reflects about 60% of the light that strikes it.

Outside the greenhouse

• Remove all remnants of shading and grime from the glazing before winter. Use shade remover or a power washer.
• Remove nearby trees if they create shadows on the greenhouse. Observe the shadows during January and February.
• Purchase a light meter and check your greenhouses for light transmittance. A few changes may increase light levels significantly.