Building a Low-Cost Greenhouse

Eventually, almost everyone who grows tropical plants comes to the conclusion that it would be very nice indeed to have a greenhouse to move their tropical plants into in winter. Not only is winter protection very desirable, but a greenhouse also extends the blooming season and helps the plants get an earlier start in spring. Yes, a greenhouse would be nice, but surely it would cost too much and take too much expertise to set up. Wouldn’t it?

Not at all. Greenhouses come in every conceivable size and price range. In the end what they do is trap the energy of sunlight inside a room that quickly warms up in response. There are more ways than one to achieve this, and they aren’t all expensive, nor do they require expert construction skills. This article will take a look at the subject of greenhouses, and make some suggestions for those of you who may be ready to take the plunge.

At the top end of the price range, there are very beautiful and functional greenhouse units that can be purchased fully equipped and installed on your property by a contractor. We’re not going to talk about those. If you’re interested in one of these, you can acquire catalogs from a number of suppliers and start your search. Some of the information below will still be of interest to you, because whether your greenhouse costs $50,000 or $500 it still needs to be able to accomplish many of the same tasks.


The Very Simplest Greenhouse Frame
One-Inch PVC Pipe Bend into a Hoop

This article will focus on “do-it-yourself” or “my-spouse-will-do-it-for-me” type greenhouses. You can build a surprisingly functional greenhouse with materials that are not expensive and are easy to work with. Home Depot and other similar home supply stores have most, if not all, of the materials you need. Since there are so many variations and so many individual circumstances, we are not going to try to give a step-by-step manual like we did with the drip system a few months ago. Rather, we will take a look at the features that any greenhouse needs to include and a few nice-to-have options as well.

One other note: This article is not about greenhouses for places where snow loads are high. That is a special circumstance that requires good design and materials that can take heavy snow loads. You can still have a home greenhouse in snowy areas, but unless you are an expert at construction, you probably will need to have a contractor build your greenhouse according to the needs of your area. This article is for greenhouses in areas that have some winter freezes, perhaps an inch or so of snow from time to time, but not blizzards and several feet of snow each winter. If you need a greenhouse for heavy snow loads, contact a company that supplies these types of greenhouses to obtain one that fits your locale.

Where To Start?

First, let us consider size. If you live in an area that is heavily regulated by zoning or subdivision rules, you should check those before you build the greenhouse only to have someone complain that it is too big or too high or not allowed at all. Assuming you are good to go, then how big a greenhouse do you need? It is a rare person who builds a greenhouse and then finds that it is too big for their needs. The opposite is almost always the case – your plant collection will rapidly grow to fill it, and you will be looking for more space. So, the best thing to do is to build as big a greenhouse as you can in the space you have available or build it with the idea that you will add onto it as needed. Greenhouses are wonderful additions to a home property and can provide many hours of pleasure while the weather outside is rotten and cold. Build it as big as you dare!

What to Build The Frame Out Of


A PVC Greenhouse Frame

There are three main types of materials used for building greenhouses at home. These are:

  1. PVC Pipe:

    Yes, common PVC pipe that is used in irrigation systems makes a fine frame for a small greenhouse. It is cheap, easy to work with, has all sorts of available connecting pieces to accommodate almost any design, and is lightweight. The light weight can be a problem in high wind areas because the greenhouse can blow away. So in windy areas, the plastic covering may need to be removed during the stormy summer months to help prevent this.

  2. Wood:

    Stronger and more stable, but also more expensive unless you have access to cheap wood. A wood-framed greenhouse requires some basic carpentry, and it helps if you have some basic power tools and a little experience working with wood.

  3. Metal:

    The strongest and longest lasting material for the frame. This would seem to be outside the reach of most people, but in reality, it’s not. There are several suppliers that provide pre-drilled metal parts, including all connectors for constructing even small greenhouses at quite a reasonable price. This is a very good option for those who want to have a somewhat larger structure, say 24 feet wide by 48 feet long or bigger.

 


A Metal-Frame is Best for Large Greenhouses

The Floor

The greenhouse floor does not have to be totally flat nor do you have to grade a perfectly horizontal surface for it. I have had greenhouses on the side of a hill and they still work just fine. It is easier to build the greenhouse “square” if the surface is level, but if you cannot make a level location, you can still build your greenhouse. A slight slope inside the greenhouse is actually beneficial because excess water will flow more quickly out of the greenhouse.


Prepping a Greenhouse Floor

We should really call this the “floor” rather than the “foundation”. It is not at all necessary to pour a concrete foundation for a greenhouse. Water run-off is always an aspect of having a greenhouse full of plants. If there is a concrete floor, the water cannot sink into the ground and must find some other way to exit the greenhouse. If you must have a concrete floor, then think through how the excess water is going to be removed from the greenhouse.

If building on the ground, lay a layer of weed barrier cloth directly on the ground, cover it with an inch or two of gravel and finish it with a second layer of weed barrier cloth on top of the gravel. This will allow water to sink into the earth, and prevent weeds from growing in the greenhouse. It also provides a decent surface for walking and working. You could skip one or both layers of weed barrier and just put down the gravel, but that is not a good solution unless the budget is too tight to do it any other way.

The Covering


Polypropylene or “Poly” Covering

Greenhouses are mainly frames covered with some sort of material that allows light to enter. The frame is the main structure, and determines the strength and shape the greenhouse will take. The covering is mostly for keeping the warm air inside and barring the cold air from entering. It is not load-bearing and is usually cheap and flexible.

Greenhouse polypropylene is the least expensive material we recommend for your greenhouse. “Poly” is graded by how many years it is expected to last in the full sun, and by how thick it is. Four-year, six mil (mm) thick poly is ideal for most greenhouses. This can be purchased in many widths and lengths from horticultural and agricultural supply companies, and of course just about anything can be ordered from online suppliers. There are also many types of rigid material that provide excellent insulation and yet still let light through. These are good for side walls and can be used to cover an entire greenhouse although they are much more expensive than flexible poly (also called greenhouse film by some suppliers).

One of the main considerations, when designing the greenhouse and choosing a covering, is rain. Rain water must be able to run off the top and sides of the greenhouse. For example, if a greenhouse is built with a flat roof and covered with flexible poly, that will be a disaster when it rains. Rain water is heavy! It weighs 2 pounds per quart. Once rainwater starts collecting in any hollow or depression in the poly, its weight will stretch the poly and create big sagging sections full of water that can’t escape. The important thing to remember is that all surfaces covered with poly must be angled so that water will flow off the surface and not collect on it. This is one of the main reasons that “hoop” greenhouses are so popular – the round shape sheds water very well. A-frame type greenhouses also work well to shed water.

Doors and Windows


Unattached Side Poly Rolled up in Summer

Besides allowing entrance to the greenhouse, doors and other openings in the greenhouse are crucial for allowing heat to escape. Greenhouses are very efficient at retaining heat while the sun is shining. The temperature in a fully closed greenhouse on a warm sunny day can reach 125°F (52°C), a temperature that is dangerous to both plants and people. Ideally, the greenhouse temperature will be kept under 100°F (38°C) and most plants grow best below 90°F (32°C). Doors and windows are the cheapest way to keep the greenhouse reasonably cool.

Another method for creating windows is to leave the side wall material unattached to the ground around the bottom of the greenhouse. Instead, cut the side wall material extra long and leave it lying loosely on the ground so that it can be rolled up and tied up during hot days. This allows air to enter and exit and helps remove excess heat. During the winter months, the excess side wall material can be held tightly to the ground with cinder blocks or other heavy material.


Simple Roof Flaps to Fold up or Down

Heat rises, so a way to let it out of the top of the greenhouse is always a good idea. Cutting simple roof flaps is one way to let heat out. Another way is automatic window openers that are set to open windows at a specific temperature and close them when the temperature drops. These do not require electricity but work on the principle of expansion and contraction of certain materials as they heat and cool. As the material expands it pushes the window open, and as it contracts it allows the window to close. Of course, you can manually open and close windows as well, but whenever it is possible to automate something in the greenhouse, you will find that it is helpful to do so.

Water

All greenhouses need water for the plants. It can be from a hose, or you can easily run some PVC pipe into the greenhouse. Working with irrigation parts may seem daunting, but it is actually very simple and straightforward. All the parts are available at home supply stores and all you do is glue them together.

Heat

Greenhouses are heated by the sun any day it is shining and even when it is cloudy. However, a cold and cloudy day may not allow for enough heat to be generated to keep the greenhouse as warm as you would like. Nights are the bigger problem since inside a greenhouse the temperature will drop to within a couple of degrees of the outside temperature a few hours after dark. That may be sufficient for areas where the nights never get below 30°F (-1°C), but for most people, some greenhouse heating is worth considering. Both electric and propane/natural gas greenhouse heaters are available to do the job. Due to the high humidity, the sunshine, and temperature swings inside a greenhouse, we have found it best to use heaters intended for greenhouses because they hold up better.

Some Optional Goodies for the Greenhouse

Over time greenhouses tend to be improved upon as the people who use them figure out what is important to them. You can start growing hibiscus in the basic greenhouse described above. It provides for warmth and water and that is what it is all about! But below are some of the goodies you may want to add as you work with your greenhouse.


Wooden Tabletop on Cinder Blocks
With Drip Water System

Tables for Plants:

Tables are great – they raise the plants to a warmer air zone, keep them out of water runoff with the potential for spreading disease, and make it much easier for the normal person to work on the plants without having to stoop or bend over all the time. Tables can be made yourself. Make table tops out of 2×4 wood frames with 1×6 wood slats, and use cinder blocks for the table legs. Plastic waterproof tables are available from many sources as well.

Drip System with Timer:

Nothing frees up the plant lover like a drip system operated by a timer. This is by far the best way to water hibiscus so that the watering is regular and uniform and does not flood the pots. Try it, you’ll like it!

Wall Fans:

One of the best ways to maintain air flow through a greenhouse during warm weather is to install one or more big box fans into one of the walls of the greenhouse. These blowouts, not in, and the pressure they create draws fresh air in from the windows and doors that are left open.


Horizontal Air Fans

Horizontal Air Fans:

These are installed high up in the greenhouse and are intended to keep a gentle current of air circulating inside the greenhouse. This helps prevent fungal diseases and also mixes the air so that the greenhouse does not end up with hot and cold layers of air during the night.

Evaporative Cooler:

Often installed at the same time as the Wall Fan, the evaporative cooling system is built into the wall opposite the fan. It consists of water dripping down special material that is collected in a gutter and recirculated. When the fan is on and the water dripping fresh air is drawn through the wet material where it is cooled 15-20 degrees. The fan draws this humid cooler air through the greenhouse and out the opposite side. This “fan and pad” system will turn a 105°F (41°C) day into an 85°F (29°C) day inside the greenhouse – a great improvement in summertime heat!

Roll up Side Walls:

Available in both powered and manual versions, these walls can be rolled up during hot days and lowered at night where they fit snuggly into a ground level track to prevent air leakage.

Grow Lights:

Grow lights are not a real necessity for most parts of the world where hibiscus are grown. However, if you live in the far north or where clouds obscure the sun much of the time, adding HID or the new LED type grow lights to the greenhouse will give your hibiscus a boost when they need it most.

Insect Netting over Doors and Windows:

One of the easiest ways to control insects is to exclude them from the greenhouse with insect netting. There are now special net cloths that have holes small enough to keep out almost every known pest, including thrips which are very skinny.

Electronic Controls:

The irrigation timer is the simplest example of an electronic control. However, fancier ones can also control the heater, the fan, the pad cooling system, and any doors, windows or walls that are powered to raise and lower. In fact, electronic controls can control anything that runs on electricity, either by timers or by sensing conditions such as temperature.


Tropical Hibiscus ‘Acapulco Gold’ Blooming in a Greenhouse

The topic of greenhouses is a huge one. We are not able to cover more than the basics in this article.

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