Heating A Greenhouse

Depending on where you live, winter can be a gentle experience with the occasional flurry sighting, or a harsh gulag lasting half the year.  While plenty can be done with an unheated greenhouse,  for many growers, a greenhouse heating system is a necessity during the winter months.

When planning a heating system for your greenhouse it helps to understand a little about how heat is both created, and lost.

A greenhouse loses heat in four ways :

Conduction : Heat moves through an object from one medium to another : from inside the greenhouse to the outside through the frame and glazing

Convection : flow of air caused by a difference in temperature : hot air rises from a heater because cold air is drawn in at the bottom.

Radiation : Heat transfers away from an object through space without the necessity of a medium. Greenhouse glazing can either absorb, store or reflect radiated heat.

Infiltration/exfiltration :  Cracks under doors and vent windows, and glazing seams allow cold air to enter (infiltration) , and warm air to escape (exfiltration). In a greenhouse, warm air escapes through cracks in the roof, drawing cold air in through openings near the floor.

How Much Heat Do I need?

Before investing in a heating system, the first step is to determine how much heat you need. While it’s possible to use formulas to calculate both the solar heat generated on a clear day, and the heat lost due to convection, conduction, radiation, and infiltration/exfiltration, there’s a  simpler formula to approximate  how much heat you’ll need to add:

(wall area + roof area) x difference in temperature between outside and inside the greenhouse x 1.1= Number of BTU’s required.

Now that you know how much heat is required to keep your greenhouse warm, it’s time to create a heating system. This may be passive heating system, and active heating system, or combination of the two.

PASSIVE HEATING SYSTEMS

Passive heating systems use water or stone to store solar energy during the day, and radiate it back into the greenhouse at night, while the hot bed method uses heat produced from the decomposition of organic matter.

Water storage tanks

Large tanks or drums filled with water, and painted black, soak up solar energy on sunny days and radiate it back into the greenhouse at night. The challenge this method is warming the areas in the greenhouse that are furthest from the water tank  ; even with circulating fans, the areas farthest from the tank can experience a dramatic temperature difference. Also, consider that tanks can freeze in winter after a few days without direct sunlight.  Thawing a frozen tank takes a lot of sunlight and heat , actually cooling the air around it as it does.

Placing the tank in the middle of the greenhouse where it is less likely to freeze is one way to address this, and, while sort of diminishing the goal of using passive solar energy, an electric aquarium heater can also be used to prevent the water in the tank from freezing.

A solar water heater can also be used to heat liquid that is then circulated through copper pipes to a heat exchanger inside the water storage tank, but again, the system only works when the sun is shining. On cloudy winter days, these systems are often supported by a wood or pellet fired stove or furnace.

Rock Heat Storage

These systems comprise insulated heating bins filled with large chunks of rock that absorb solar heat blown across the medium by fans during the day, and then circulate the warmth back into the greenhouse at night. While they require little to no maintenance once installed, rock heat storage systems do require a substantial amount of volume:  every square foot of greenhouse glazing, 3 cubic feet of rock is needed. While large wire baskets can be used to store the chunks of rock, a more efficient way is to place 3- 4 feet of rocks beneath an insulated floor.

Hot Beds

For greenhouses where growing beds are used, the energy created by decomposing manure can be used to warm the grow environment around the plants. Simply dig out the beds down to a depth of about 3’ and fill them with horse manure and other compostable material like leaves and grass clippings, back to about a foot below the top of the beds. Then top that off with garden soil. As the organic matter breaks down, the temperature will increase to about 160 degrees over the course of about 5 weeks, remaining there for about two months before gradually cooling down to about 80 degrees. 

ACTIVE HEATING SYSTEMS

Systems that draw on sources other than the sun, including coal, wood, wood pellets, gas, oil and electricity are referred to as active heating systems.  While certainly more expensive to operate than passive heating systems, active heating systems afford much greater control of the temperature in the greenhouse, and allow for much higher temperatures.  Insulating the north wall and opting for multi-wall glazing will help offset the cost of running an active system.

Electric Heating

Clean and simple to use, electric heaters of course require that your greenhouse is wired for electricity. While electric heaters themselves don’t have to cost a lot, the cost of the electricity they use can add up fast in winter, especially if the greenhouse is poorly insulated.  Costs can be controlled by maintaining a temperature between 40 and 50 degrees during winter, which is warm enough to keep most plants healthy, without producing sticker shock when you open the power bill.

Wood and Wood Pellet Burning Stoves

While larger greenhouses are commonly heated with 55 gallon drum stoves vented with a chimney, a standard wood burning stove is adequate to heat most hobby greenhouses.  Besides the need for constant feeding, which means frequently opening the door and allowing heat to escape, the logs can also introduce insect pests into the greenhouse.  A better, but more costly option is a wood burning furnace placed outside the greenhouse. This means no heat lost opening the door, or unwanted insect pests, and it also eliminates any risk of carbon monoxide accumulating inside the greenhouse.

Coal Stoves

While they share many of the same benefits of wood burning stoves, including the ability to generate a lot of heat, they must also be monitored and refilled.  Consider where you’ll store the coal and dispose of the coal ashes.

Gas Heaters

Usually designed to run on either propane (LPG) or Natural Gas (CNG), gas heaters are easy to use, and safe as long as they are operated properly.  If the unit doesn’t have adequate ventilation or air intake, the oxygen in the greenhouse can become depleted, creating a hazardous environment for people. Propane leaks can also damage your plants. Models that require no outside power source are ideal, especially as a back up to an electric heating system.

Egg Production: How It Works

With a puzzled look, the mailman has just handed over your new baby chicks. Even the least observant among us could readily see that it will be a while before these peeping fluff balls begin laying eggs. But how long will that be? And how does it work, anyway?

When a pullet (female chicken) is born, her body holds as many as 4000 ova, which are undeveloped yolks.  When she begins laying – usually around 24 weeks – the eggs will initially be small, with one laid only every 3 or 4 days.  At around 32 weeks of age, laying will increase to about two eggs every three days, and the eggs will reach a normal size.

One at a time, the ova will grow into full size yolks, and drop into the oviduct : a 24” long tube that surrounds the yolk in egg white and encloses it in a hard shell.  The process  takes about 24 hours, and which point the egg is ready to be laid.

The total number of eggs a hen can ever lay is limited to the number of ova she was born with, but few chickens will live long enough to lay all 4000. ; most laying hens will produce somewhere around 1000 eggs in their lifetime.

In her first year, a healthy laying hen produces about 250 eggs, and at around 18 months of age, the molt process begins.  During the molt, which happens once a year – typically in the fall- old feathers gradually fall out and are replaced with new ones.  The process takes about three months, and few  if any eggs will be laid during this time because a hen will divert all of her energy to growing new feathers.

With the molt process complete, and donning her chic new coat of feathers, a hen will begin laying again. While the eggs are larger, typically the amount of eggs will taper to around 200 eggs per year.

Ultimately the number of eggs a hen produces depends on a set of factors including breed, health, living conditions, and weather.  The ideal temperature range for optimum laying is between 45 and 80 degrees, and laying tapers off when it is below or above that range.

Because shorter day lengths will eventually dip below 14 hours, even healthy hens will cease laying in Winter. An easy solution to this is to install a 60 watt light bulb in the coop, combining the light with natural daylight to provide at least 14 hours of continuous light.

Mahoney Family's Gable Cedar Cold Frames

The winds that blast across the Mahoney’s lakefront property are no joke. Gusts that routinely reach 40 MPH topple wood patio furniture like dominoes, and quickly weed out all but the strongest landscape plants and flowers.

Their organic kitchen garden is mere feet from the water’s edge, and right on the front lines of this daily barrage.  Despite a generous automated watering schedule, the soil can quickly dry out from exposure, and even in ideal spring conditions, establishing tender young seedlings can be a challenge.  

The same gale that can be a blustery but manageable force in warmer months is much less forgiving when it comes with the bite of winter. 

Mahoney Family's 5'x8' Cedar Gable Cold Frames from Microfarm Organic Gardens on Vimeo.

A healthy assortment of established cool season varieties in the Mahoney's garden was beginning to flag, and Microfam’s Gable Cedar Cold Frame design was the answer. Sturdy cedar framing fitted with Agribon AG19 fabric, will allow water , air and light to pass through, while protecting the established cool season plants from wind gusts and frost.  The gable design sheds excess water and debris, and features two hinged lids that can be propped open for easy access to the garden.

Not only can the Microfarm Gable Cedar Cold Frame design protect fall gardens well into winter, but they also give growers a head start on early spring sowing, helping warm the soil for improved germination, and protecting young seedlings from insect pests, wind  and frost.

During the summer months, the design’s hinged lids can be easily removed and stored until they are needed again.

Interested in protecting your organic garden with a Microfarm Gable Cedar Coldframe? click here to get started.

Greenhouse Glazing

Like choosing the color you paint your house, or the new countertops in the kitchen, greenhouse glazing is a decision you’re may have to live with for a while. Once the greenhouse location has been selected, the clear material used to cover the greenhouse, or glazing, should be the next selection in the design process. The type and weight of glazing material will then dictate the strength requirements and design of the greenhouse’s frame.

Building a high tunnel style greenhouse with a PVC frame to get through one winter? A roll of lightweight polyethelene sheeting will do the trick.

Creating an elegant lean-to structure built onto the south facing side of a luxury home? A high performance, multi-pane glass would be more appropriate.

But whatever the greenhouse glazing material, ultimately the goal is the same: To create an indoor grow environment that allows for control of temperature and humidity while allowing natural sunlight to reach the plants. While it is certainly feasible to use supplemental lighting inside a greenhouse, most growers will rely on sunlight, thus the greenhouse should be directionally located to receive the maximum about of available sunlight.

Though solar radiation has a broad spectrum of wavelengths, studies have shown that only wavelengths between 400-700 nanometers are useful for photosynthesis in plants. Known scientifically as photosynthetically active radiation (PAR), radiation in this range is about the same as what is visible to the human eye, and inevitably, sun light will lose some of its radiation as it passes through glazing into a greenhouse. Shading from the actual greenhouse frame, benches, and shelving can also reduce the amount of light that reaches plants by as much as 30 percent.

In addition to light transmission, several other variables should be carefully considered when selecting greenhouse glazing:

Maintenance : how easy is it to clean dirt, leaves, bird droppings, etc. from the glazing material. How difficult is it to repair damaged glazing?

Life Span: How many years will the glazing material perform at an optimum level before it must be replaced?

Structural Value: How strong is the glazing material, and how substantial must the greenhouse frame be in order to support it?

Insulation(R – Value): How effective is the glazing material at retaining heat? R value is a measure of a material’s thermal resistance, and in theory, the higher the R-Value number, the better the material’s insulation value.

Degradation: How quickly will the glazing material break down and lose performance from UV exposure?

GLASS

The first and only greenhouse glazing option for many people, glass has many advantages. It’s plentiful, strong, long lasting, and has high R-value and as much as 90% light transmission. It comes in different thicknesses, and is made with high performance coatings that allow even more control of the conditions inside the greenhouse.  It’s the ideal glazing in so many ways, but it does have a couple of drawbacks that should be considered. Glass is fragile, and sometimes a broken pane is unavoidable. At the very least this can make for a tedious morning spent picking pieces of glass out of  garden bed or gravel floor, and worst this can post a significant risk to personal safety. Another consideration for glass glazing is weight.  Single pane glass isn’t especially heavy, but it adds up quickly when building with double or triple pan high performance glass sheets, requiring careful consideration of the strength of the frame design.

Single Pane Glass (R-Value 2-8)
This plentiful and inexpensive glazing material offers about 90% light transmission, but should be handled with care and considered carefully along with other, less fragile(dangerous) glazing options. While 3/32” and 1/8” panes are commonly used, investing in thicker,  ¼” single pane glass will greatly increase impact resistance. Remember that with thicker glass, of course, comes more weight, requiring a more substantial greenhouse frame capable of supporting the heavier load.

Laminated Glass
Made by sandwiching a layer of plastic or resin between two layers of glass. It’s frequently called safety glass or shatterproof glass because the plastic layer holds fragments together if the pane is shattered.

Tempered Glass
Created by controlling the cooling phase of production, tempered glass can resist about four times the impact of regular glass. In the event that it does break, tempered glass shatters into tiny pieces, called dicing. While it’s a tedious process to clean up a myriad of tiny bits of glass, the danger of being seriously hurt by a large shard falling from overhead is dramatically reduced.

Double and Triple Pane Glass
Two or three layers of glass with an air space in between them are held apart by a special, moisture absorbing  beading.  While R-value is improved, light transmission is substantially reduced with double or triple paned glass glazing. Eventually the seals between panes will break down allowing moisture between the panes, producing a clouded effect and further reducing light transmission.

Low E and MC Low E Glass
Frequently used for solariums (glazed roofs) and sun rooms ( solid roofs), tempered low emission, or low e  glass is available as an upgrade by some high end greenhouse manufacturers. It allows heat to enter the greenhouse, helps retain heat inside, and also blocks UV radiation from the sun.  Tempered Low e and MC low e glass can reduce light transmission to as low as 65%, also blocking much of the light spectrum that plants need for vigorous growth, making it a better choice for a solarium application than a greenhouse.

FIBERGLASS (R-Value 1-4)
Available in flat or corrugated sheets, fiberglass is flexible and strong and delivers about 90% light transmission. It’s inexpensive, lightweight, easy to install, and impact resistant.  Besides a more homely appearance than other glazing material, however, fiberglass tends to yellow with age, and is highly flammable. Expect about 8-12 years of useful life before it will have to be replaced.

ACRYLIC (R-Value 2-6)
Often called by the trade name Plexiglass, Acrylic has some key advantages in greenhouse glazing applications. Double walled acrylic sheets have excellent insulation value, and yield about 85% light transmission. Acrylic sheets are structurally stronger than fiberglass, and are easier to clean and maintain. The useful lifespan is similar to fiberglass, though : about 8-12 years. Keep in mind that cheaper acrylic material will quickly yellow and cloud from UV exposure. Premium acrylic panels designed for use in greenhouses are often treated with a UV resistant coating and come with a 10 year warranty against discoloration.  The nature of acrylic material makes it easy to cut, and bend into custom shapes with special heating tools, but these same properties also mean it is also easily scratched and cracked. Use care when drilling, and leave protective films in place until installation is complete.

POLYCARBONATE (R-Value 3-6)
Like acrylic, polycarbonate is sold in twin wall sheets specially made for greenhouse applications. An exterior coating that blocks UV light helps prevent discoloration and the sheets typically come with a 10 year warranty against fading. Often the material will perform for as long as 15 years however, and polycarbonate is very shatter resistant, making it a safer choice where kids or seniors may be present. It has excellent impact resistant to damage from objects like hail stones, tree branches, acorns, etc.  Twin wall polycarbonate Sheets are sold in 48” and 72” widths, in thicknesses from 6mil to 12mil or higher. Although polycarbonate is a lightweight material easily handled by one person, it has significant structural value allowing for greater spans between rafters and reducing sun light exposure lost to shadows.

POLYETHELENE  & PVC SHEETING (R-Value 1-3)
Both inexpensive and easy to install, PE and PVC sheeting also offer excellent light transmission, and are still the greenhouse glazing of choice for professional nursery growers. With a useful lifespan of just a few years, and low insulation value, plastic sheeting is not a long term glazing solution for the typical home grower, and despite even the most meticulous installation few would consider it an attractive enhancement to a residential backyard landscape.  

Urban Homesteading on WFAE's Charlotte Talks with Mike Collins

Composting toilets. City livestock permits. Goat husbandry. Rain water harvesting. These and other urban homesteading topics and were explored on-air live , and with a refreshing dose of humor on WFAE's Charlotte Talks Show with Mike Collins on 1.9.15.

In the Charlotte studio, Microfarm Organic Gardens founder Matt Kokenes joined Dr. Dave Hamilton and Laura Deyenes of Wish We Had Acres Farm, while Los Angeles based author Erik Knutzen got in on the live discussion via telephone.

According to the show's producer,  streaming was high during the brodcast, and positive feedback about the day's topic quickly amassed on the station's social media pages. Was it really that interesting? 

click here to listen to a recording of the show.