Dirty Little Greenhouse Secrets Revisited

The primary components of a working greenhouse are damp and dirt.

Dirt is a given. Potting soil does not stay in pots. You know those fancy pictures you see where everything is spotless and the floor is white? Count the plants: a few perfect ones for decoration. If those are your aspirations, you want a conservatory with a writing desk and a place for tea, not a greenhouse.

I do a lot of hand watering. Do you heft your pots to see if they're lightweight? A heavy pot may be water soaked and not need a general spraying with the rest.


Cachepots are not just for decoration. Plants in a plastic pot get top-heavy. In a cache pot they're secure and can be lifted out to water and drain, repot, dab with an alcohol-soaked q-tip for bugs, or whatever. The space between a thin plastic pot and a cache pot helps keep cold drafts off the plastic pot. A recycled glass jar inside a cache pot is a good place to root cuttings. A plastic container sitting in a ceramic cache pot is bound to have warmer roots than in a thin plastic pot sitting on a wood shelf.

Thermal mass helps mediate daytime heat to nighttime cold. There is not a way to keep a greenhouse above freezing in a cold winter without supplemental heat in some form. The easy, less expensive way is a couple of heaters from the big box store. Be sure they have high and low settings and a thermostat. You must use the low setting if you want your heater to last as long as your greenhouse. It takes some trial and error on cold nights to get the thermostat set 'just right.'

Behind the center group of plants is a piece of styrofoam that was packing for something we ordered off for. It is recycled as insulation behind this group, on the north side of the greenhouse where it reflects light coming from the south and west.

Heaters take space. Remember I said a greenhouse needs two? Better one still running if one goes out than a big heater that hasn't a back up when it fails. I didn't mention adequate wiring; it is important. Here's the thermal mass part: There are charts online that give the information about what substances are most useful in holding heat generated by the sun. Water is most efficient. If  you can get some barrels for water, hooray! The barrels can have a shelf on top to offset the floor space they take up.

Each plastic gallon jug of water used will help a little bit. I fill mine and line them up in front of the heaters, usually about 8 or 10 jugs for each heater. Heaters and water sit on the walkway. When the weather warms, heaters go back in the tool shed and I water plants from the jugs and put them in the shed until the next cold spell. The hot air blowing on the water jugs warms them and the heat is given off more slowly than the hot air all rising to the ceiling as the heaters blow on nothing. That's my theory, you may  not agree.

What else helps with heat mediation? Remember those cache pots? Ceramic material, bricks used as plinths, stones, pieces of concrete -- all these are helpful to hold heat. Sun shines on them during the day; later when heat is needed as the air cools that heat is given up.

I read this week about heating with compost. There isn't that much room in my greenhouse. I don't know of an feasible inexpensive, nearly free way to heat. I address keeping plants from freezing here, affordable but not cheap. I keep my orchids in the house where it's warm.

 

Divided the Guzmania. Two plants  replaced the old one; both those had pups

around them and now the pups have pups. Bromeliads have life cycles.

Too much is never enough. You'll never have enough space or too many kinds of plants. You may eventually have too many of one kind. Share with others.

Plants tend to grow. The little Birds Nest Fern of a few years back now takes up nearly 7 square feet of GH floor space. Sometimes I let plants bunk together, like Purple Heart and Persian Shield in a container together, or Foxtail Fern and Chlorophytum.

 

A 2" pot of Burro Tail yielded 7 pots full 

in under 3 years. Plants grow.

Frog Poop. The best pest control in a greenhouse are little toads, peeper frogs and lizards. They do donate their little share of fertilizer and it isn't always pretty. Sometimes when I add water to the cup of a bromeliad, a tiny green frog emerges. Cachepots are places for frogs to hide. The greenhouse is alive.

Persian Shield in bloom in winter.

It isn't really work. There are plants that find just the right time to bloom when the days are short and they're hanging out in your greenhouse. I would never otherwise have seen blossoms on Burro Tail Sedums or Mistletoe Cactus and probably not on White Shrimp Plant and Persian Shield.

Burro Tail Sedum winter blossoms

Rhipsalis blooms, midwinter

How I Heat my Greenhouse

The Internet abounds with advice on heating a greenhouse. This is just a summary of how I keep plants that are not overly cold-tender through the winter.

Buckeye on Burro Tail Sedum this morning in the GH.

Before you read about how we heat, you need to know these facts:

1. We grow in US zone 8b. Our primary greenhouse concern is not heat but keeping plants cool in the daytime when the sun is shining, even in winter. Our nighttime lows might reach 15º once in a great while but never for prolonged periods. Below-freezing temps quickly rise at daylight. We had two recent freezes with temps that hovered about 31º F.

2. There are sites with greenhouse heaters for sale. We used what we had, ordinary space heaters with a trial period to see if they worked

3. There needs to be a back-up plan. Ours is a kerosene heater. I am thinking about assembling a brick rocket stove as a trial on the greenhouse floor which is brick broken concrete and river stones . Using old chimney bricks it might be a no-cost solution in an emergency.

4. My heaters are on separate breakers in a greenhouse wired for electricity with its own electrical panel. Never try this using extension cords.

5. Orchids grow in the house in a sunny window. I don't put anything in the greenhouse that wants nighttime temps above 50º -- that's for commercial growers or serious hobbyists.

6. Not professional advice, this is how I keep my plants from freezing using what's at hand.

My greenhouse is filled with as much dense thermal mass as I can find and put in there. Thermal mass includes brick, concrete, water and stone. These materials absorb heat and give it up slowly during the night. Note: Thermal mass will not heat your greenhouse, it slows the radiation of heat from the greenhouse when the sun goes down. Eventually the greenhouse is going to be as cold as the air outside.

 Concrete blocks are of two types, dense concrete and those lightweight blocks that have air forced into the manufacture. Dense blocks are best. They make nice plinths for big pots, hold up benches and could even be filled with soil and used as planters. As far as I can tell, bricks is bricks, handy for many uses. I use stones as decorative material and do not worry about whether one kind is better.

Water in the greenhouse might take the form of huge tubs for hydroponic growing, not in my league. I have 4 fifty-gallon barrels of water that support a plant table. We tried all kinds of fancy tricks using pumps and black hoses. Those have been abandoned and the barrels just sit, covered in black plastic and a burlap skirt for aesthetics.

Now to the heaters. These are space heaters we already had, regular little space heaters with a fan, a thermostat and two settings, high and low.

Here's the important part: We run these on the lower setting. The difference is typically 600 watts on low and 1500 watts on high. By using two heaters on the low setting we use about the same electricity. Less current pulled through the wires means the heaters may last longer. If one fails, there is still the second putting out some heat rather than a total fail on a cold night.

 

It takes trial and error to get the setting just right so that the heaters come on at just the right temperature to keep the inside far enough above freezing for plant survival.

Why don't I just use a greenhouse thermostat? I would need one for each circuit and they are not cheap.

Otter Tail Power Company in Fergus Falls, Minnesota has a great page on using electric space heaters.

Celery from a Stalk Bottom, still growing.

Bouquet I picked before the freeze.

Linking to Tootsie Time's Flaunt Your Flowers