Persephone

The Persephone period is the time of year when growing light hours fall below 10.  It is named for the time of year when the Greek goddess Persephone visits the underworld and her mother Demeter is stricken with grief.   

Below 10 hours of growing daylight, plants generally become dormant and cannot put on new growth or produce fruits or flowers.  This is true, regardless of the ambient temperature.  For example, even in a greenhouse where the temperatures are regulated, in the dark of winter, plants generally do not grow during the Persephone period without supplemental light.

 

In Virginia: 

On October 11th, sunrise is 7:25 am and sunset is 6:25 pm for a total daylength of approximately 11 hours.  

The threshold of 10 hour days is typically when plants stop growing and developing.  On October 31st, daylength is approximately 10 hours.  Actual daylight hours, including twilight, is about an hour longer.     

On December 21, day length is 8:23 hours long in Virginia, the shortest day of the year.  On February 10th, daylength again crosses the 10 hour threshold when the most cold-hardy plants begin growing again.

Note that this is different from temperature patterns, where February is typically the month of the year with the greatest potential for winter snow. 

 

Greenhouse Modifications

 OK, so you've got your greenhouse kit waiting to be assembled.  What follows are a few modifications that will improve the experience, recommended by gardeners who have lived with and experimented with their greenhouses over a few seasons.

 

Modifications #1 

1.  Use some kind of treatment on the lower wooden panels.  These are the panels that will see the most wear from rain, dust, mud abrasion.  Even if the wood is already cedar or pressure treated,, use some kind of weathering treatment to extend the life of the whole greenhouse.  Water seal, polyurethane, whatever you can get by the gallon.

2.  Ventilation is key in the summer.  The inside of the greenhouse will become an oven if the doors and windows remain closed.  Make sure that all ventilation remains open and unobstructed.  Find a way to secure doors open.  Are there temperature regulated windows or louvered vents that can be used in shoulder seasons to regulate the temperature?  This is key in June when the nights would benefit from the greenhouse being shut, but daytime temps can spike to dangerous levels when the sun is beating down.  

3.  Strategies for the height of summer - July and August.  

• Consider covering with shade cloth at the hottest months.  Even partial covering, of the back half for example, can give you some escape from the intensity.
• Find ways to open as many panels as possible.  
• Put some of the lower panels on hinges to fully open but add cabinet hardware to seal securely in the winter.  
• Add additional screened vents that can be closed or sealed with plastic in the winter, but will pull more air into the interior in the summer.  

 4.  Interior plumbing.  

• Add a hose bib, 
• draining into a basin inside to rinse off vegetables and water seedling trays.  
• Supply water with a garden hose on the outside, but use actual plumbing on the inside.  
• Consider a  rain barrel collection system off the roof 

 

5.  Electricity for working in the evenings during the fall and spring.  Use a heavy duty extension cord to run temporary power to the greenhouse.

• Overhead lights 
• Ventilation fans
• Diesel heater
• 
  

6. Heat.  What can be done to keep the interior above freezing overnight:

7. Lighting.  

• Overhead lights 
• seed starting lights
• Solar exterior lights. 

8.  Wire frame shelves for seed trays 

9.  Plant some flowering around the greenhouse, and even inside to attract pollinators.

10.  For deep winter cold,  plan for layers of frost protection.  

• Seal the gaps in the outside envelope of the greenhouse. Use a clear silicone to calk the seams.
• Consider adding bubble wrap on the roof, or reflectix around the solid perimeter walls for extra insulation.
• Create a smaller area within the greenhouse by enclosing it with plastic sheeting.  This could be a tent within the greenhouse, or section off one bench or one wire shelving unit.  Make bags out of agricultural fabric with a draw string around the opening that you can pull tight over individual pots.
• Within these small microclimates, you can introduce smaller, more efficient elements of heat, such as a seed-starting heat mat, or C7 string lights that can heat a much smaller area without a heavy electrical demand.
• Place water barrels within the zones to add thermal stability.  They absorb heat during the day and radiate it again at night time when outside temperatures are the lowest 
• Create "propagators" out of clear plastic tubs that will hold a seed starting flat or 10x20 tray. Place a heat mat in the bottom as a heat source.
• The use of a space heater with the thermostat set to a minimal 40 F can be effective to keep plants from freezing without being excessively expensive.  This will not produce tomatoes in January, but it may allow some already cold hardy winter greens from pushing through an outside freeze.