As part of http://prairefairymadison.com our goal is to grow native plants for Southern Wisconsin residents to plant in their gardens. The time of year that most people are interested in plants is the springtime, around Mother’s Day. Growing native plants so that they are ready by this time is difficult for a number of reasons:
- Most perennials require a few months for germination and growing before they are ready for sale. As such, we would need to start germination in January. Germination requires temperatures around 60F for optimal growth.
- Wisconsin winters can be cold! Climate normals for Madison show an average low temperature of 11.8F in January. But, the most important metric is the lowest temperature. I downloaded daily temperature readings for Madison and parsed it with a simple python script. There were 35 days in the last 10 years below -10F, with the lowest at -26F on January 30, 2019.
Lots of plants
For the sake of design, we plan to grow 4500 plants to have available in May. They are grown in 10×20 (10.94″x21.44″ real size) flats with 32 plants per flat, so roughly 140 trays.
Greenhouse
One option for growing these plants in winter is to build a greenhouse and utilize the suns power. A Bobcat greenhouse from Rimol is 16′ wide. Assuming we can fit 3 rows of trays across 16 feet with 3′ paths between:
140 / 3 ~= 47 rows x 10.94 in = 43 ft long. So, our greenhouse would need to be 16′ x 43′ (minimum). The Bobcat greenhouse comes in 48′ length, so we’ll design to 16′ x 48′.
Now we need to design the heating required for the greenhouse.
To calculate the area of the greenhouse walls (gable frame, 11′ high with 5′ side walls), I used http://www.littlegreenhouse.com/area-calc.shtml and came up with 1760 square feet.
Now referencing http://www.littlegreenhouse.com/heat-calc.shtml and assuming double-layered inflated poly walls, we get a maximum BTU requirement of 105952 BTU to keep the greenhouse at 60F when it’s -26 outside.
Gas heater
Gas heaters are 80% efficient normally, so the required heater input is 132440 BTU. I wrote a python script to calculate the fuel cost for January – May with the following assumptions:
- Each night is 12 hours long
- The night is assumed to be the same temperature all night, the measured low.
- This is a compromise since calculating heating during the day is very complicated given the heating effect from the sun. Thus, day heating is ignored and night heating is worst-case.
- 91,000 BTU per gallon LP
- $2.5 per gallon LP (typical winter cost)
These are the calculations:
- 2014: 107368800 BTU 1179 gallons 2948 dollars
- 2015: 97777680 BTU 1074 gallons 2685 dollars
- 2016: 86227680 BTU 947 gallons 2368 dollars
- 2017: 82383840 BTU 905 gallons 2262 dollars
- 2018: 95449200 BTU 1048 gallons 2620 dollars
- 2019: 98442960 BTU 1081 gallons 2702 dollars
- 2020: 86504880 BTU 950 gallons 2375 dollars
- 2021: 92566320 BTU 1017 gallons 2542 dollars
- 2022: 94636080 BTU 1039 gallons 2598 dollars
- 2023: 83658960 BTU 919 gallons 2298 dollars
- 2024: 78521520 BTU 862 gallons 2155 dollars
For a typical winter consuming 1000 gallons of LP, this equates to 5.6 metric tons of CO2 emitted (1000 * 1/42 barrels per gallon * 236.0 kg CO2/barrel * 1/1000 kg/metric ton). That’s equivalent to driving about 10,000 miles with a car that gets 22 mpg.
Electric Heater
Electric heaters are nearly 100% efficient. Converting 105952 BTU/hr to kW results in 31.05 kW. We have an existing 120V/60A service coming out to our out-building, but that can only handle 7.2kW. Our house has 200A service and the greenhouse would require 130A itself (31000W/240V) so we could theoretically run the electric heater but at great cost.
Indoor Growing
Growing indoors is an option, at least for during the coldest parts of the winter for germination.
How much light do we need? A good comparison is the sun. I wrote a python script to calculate the solar irradiance for a given date, using pysolar and astral for our location. Every 30 minutes between sunrise and sunset (from astral), it calculates the sun angle from pysolar. Assumptions include:
- The conversion from W/m2 to PPFD (umol/m2/s) is 2.1 in the PAR spectrum for the sun, which was obtained from this reference.
- Each day is clear skies.
The output of the script for the first days of each month from January to May:
- Total PPF on 2024-01-01 00:00:00: 44.01 mol/m²
- Total PPF on 2024-02-01 00:00:00: 52.22 mol/m²
- Total PPF on 2024-03-01 00:00:00: 61.94 mol/m²
- Total PPF on 2024-04-01 00:00:00: 70.15 mol/m²
- Total PPF on 2024-05-01 00:00:00: 74.82 mol/m²
A LED strip such as this consumes a maximum of 48W to produce (210 lumen/W * 48W) about 10,000 lumens. Using this calculator this comes out to 178 umol/s (assuming high CRI LED at 4000k). Each plant tray is 10″x20″, and the LED strip is 1.2″ x 16″, so we can assume to use one LED strip per tray. Using this PPF to PPFD calculator, assuming 2’x1′ target area with 50% of the light reaching the target, we get 479 umol/m2/s.
If the sun in January is 44.01 mol/m2 (44010000 umol/m2) then we can find how long the light needs to be on. 44010000 umol/m2 / 479 umol/m2/s = 91880 seconds. 91880 seconds is about 25 hours. This s
For April we should expect roughly double,
If we have 140 light strips each consuming 48W, we use a total of 6720W.
A wooden rack could be constructed that houses 5 rows of trays, 6 trays per row (5.25′ long). This power supply can power 6 LED strips (one row). With 30 trays per rack we need 5 racks to supply our target 140 trays, though the 5th rack would not be fully populated.
