Straw Bale Garden “Greenhouse” Provides Two-Week Or More Head Start

“One of the greatest benefits of this Straw Bale Garden system is that it can give you a head start of two weeks or more on the growing season,” declares Joel Karsten, author of Straw Bale Gardens Complete. “The internal heat created by the decomposing straw is a big part of this,” he adds, “But to truly take advantage of the opportunity, be sure to add the ‘greenhouse’ element I’ve developed for a standard SBG layout. The best time to build your retractable greenhouse cover is right after you finish planting.”

Karsten uses a custom-fit length of 2 or 3 mil (1/1000 of an inch)-thick sheet of clear polyethylene to make the greenhouse cover which is pulled between the two wires at the 10-inch height on the posts that are part of the trellis structure on either end of the row of straw bales in a standard SBG layout. “The poly cover will hold in the heat being generated by the ‘cooking’ bale, and keep the new transplants and seedlings warm and toasty,” he says, “While the nighttime temperatures and even some daytime temperatures can be damaging to tender plants put out too early, it is virtually impossible to incur damage under the tent covers with the little compost furnace running underneath the seedbed.”

As the plants grow, they will begin to touch against the poly cover, requiring the move of the cover up to the 20-inch wire level of the trellis structure. As the higher level will considerably increase the volume of air to be heated for continued plant protection, do not move the cover from the lower level until plant growth touching the plastic makes it necessary.

Understanding Air Temperature

Karsten warns that on warm days, especially when the sun is shining, temperatures under the covers can get very warm, “So be prepared to slide the cover to one end and clip it behind the end post at the start of any day with a weather forecast calling for at least 65 degrees F. and sun. Keep the covers off until the forecast calls for temps cooler than 45 degrees F. at night and then pull the covers back over the top. Keep the covers on until the forecast calls for daytime temperatures above 65 degrees F. This system seems to work very well, and you’ll be surprised at how few times you will actually have to pull the covers over or remove them in one growing season. Keep in mind that the covers are also there to protect the seedbed from any really hard rain, or from hungry wildlife looking for a salad bar.”

As long as air temperatures remain in the 30s or warmer, Karsten says there is no need to tuck in the poly covers, but if the forecast “is to reach down into the 20s, then tuck the poly cover sides tightly under the strings, and wrap the ends snugly around the posts to prevent wind from getting up under the covers. If the strings are too tight on the bales, and you are not able to tuck the plastic behind them, just drape the poly over the top and tie a long piece of rope or twine all the way around the bottom of all the bales on that row and on top of the poly sides. Tie the rope nice and taut. The bales will stay cozy and warm, and your seedlings will think they are inside a comfy greenhouse, even though the temperatures outside may fall through the 20s.”

Although most traditional greenhouses require either thermostatically-controlled oil, gas or electric heaters to maintain set temperature margins, Karsten says, “Our straw bale greenhouse requires no supplemental heat because it is very rare that the temperatures will ever get low enough to cause any problems. As the bacteria are digesting or composting the straw, they give off a great deal of heat. It is rather common to see a reading that is 15 to 30 degrees warmer than the outside air temperatures, even on a partly sunny day.”

Considering Materials to Control Temperature

But Karsten has an emergency plan if winter should unexpectedly return and temperatures are predicted to drop below 20 degrees F. “I would recommend adding another layer of poly film above the one at 10 inches, using the two wires at 20 inches,” he notes, “This sandwich of two layers of polyethylene film will trap a layer of air in between. Trapped dead air has an incredible insulating capacity to protect from cold temperatures, and with the bacteria heater running inside the bales, your mini, straw-bale greenhouse will stay toasty warm.”

In areas where the winter nights rarely drop below 30 degrees F., Karsten suggests, “Row cover cloth is an alternative to the clear poly film that is worth considering. The poly can actually get too warm on sunny days. The row cover cloth breathes better and yet will retain much of the heat from the ‘cooking’ bales around the plants. Row cover cloth normally comes in 6-foot width, so if you simply get enough to cover the length of your row of bales, you will have plenty. It is reusable for many years, and reasonably priced, so it might be worth it to keep some around for those years when the weather warms up early and the poly covers would be too hot.”

Karsten also advises that gardeners keep bird netting on hand to go on when its time for the poly covers or row cover cloths to be removed, not only to discourage birds, but also rabbits and deer from damaging the garden. “Normally I like to drape the netting over the top of the top wood rail on the trellis and let it fall down to the ground on both sides,” he adds, “Netting, much like polyethylene, comes in many different sizes, but I would recommend the 14-foot width and whatever length your garden calls for. I like the ¾-inch mesh; it works best for birds and rabbits, and keeps the deer away as well.”