Greenhouses: How to Choose & Where to Buy
Greenhouses enable gardeners to ensure productive crops regardless of the weather or growing region. Greenhouse gardening used to be the sole domain of commercial plant producers who require a protected, controlled environment to grow flowers and food crops indoors off-season, or to raise starter plants and shrubs for resale. More recently, due to modern materials and new construction techniques, many new types of greenhouses have become available for sale to anyone interested in gardening. In fact, it's become common today to see small greenhouses in residential backyards.
Homeowners and gardeners who are thinking of buying a greenhouse have two decisions to make: Do we need a greenhouse? If so, with so many choices on the market, how to choose which greenhouse to buy?
Do you need a greenhouse?
Gardening in greenhouses has become increasingly popular due to three trends which affect gardeners everywhere:
1. New materials have made greenhouses less expensive and longer lasting
Greenhouses have become much more affordable recently because of new materials which lower the cost and increase the lifespan of modern greenhouses. Manufacturers have responded to these developments by producing a wide variety of new greenhouses of all sizes and styles, and in a wide range of cost.
2. Food security is enhanced with a home greenhouse
To many people, faith in food security is shaken by each new report of outbreaks of sickness from various produce providers. Also, the cost of food, especially produce, has risen considerably due in part to transport costs, and it's getting to the point where it may be healthier, less expensive, and more secure to provide your own food for the family.
3. Climate change brings uncertainty to outdoor growing conditions
The frequency and strength of seasonal storms is increasing, as predicted by climate scientists. Gardeners across the world are losing crops and topsoil to unusually strong weather accompanied by heavy rains which flood ground crops and wash away topsoil where most soil nutrients reside. In our garden, we recently had a major 'November' storm arrive two months early, breaking many of our tomato plants, knocking over the fall brassicas (cauliflower, kale, broccoli, brussel sprouts and cabbage), soaking the winter squash which should have been curing in the early autumn sun, and throwing our fall fruit harvest off schedule. As gardeners, we now have the added risk of unexpected weather events. Growing our principal crops in a greenhouse, or starting crops early in a small greenhouse, is our best assurance of a fruitful harvest in uncertain times.
Benefits and drawbacks of greenhouses for gardening
Besides the trends mentioned above, there are many practical benefits, and some drawbacks, in using a garden greenhouse to start or grow crops under shelter.
There are some drawbacks to gardening in greenhouses which can be mitigated by choosing the right greenhouse and greenhouse accessories for your growing region.
What are the different types of greenhouses available on the market?
Choosing which greenhouse is right for you can be a daunting task, since buying a greenhouse is an investment which requires some research. To help narrow down the choices among the many greenhouses for sale today, you can choose from among these general types of greenhouses based on your garden needs.
Cold frames – These 'mini-greenhouses' are designed to be set directly on a garden bed for direct seed germination and for hardening off transplants. Cold frames are primarily used in early spring to get seedlings to sprout and take root where they would otherwise perish due to frost and cold spring rains. Cold frames can be moved to different beds to follow the spring planting schedule. Visit Eartheasy's online store to see a wide variety of Cold Frames.
Starter greenhouses – These small and mid-size greenhouses are very popular for propagating seeds and starter plants which will be transplanted outdoors to garden beds. The starter, or hobby, greenhouse also doubles as a potting shed, where garden supplies can be kept under cover, transplants can be repotted, and harvested crops can be brought in to cure and be processed for winter storage. These greenhouses often have clear or semi-diffused coverings, since the direct light favors seed propagation. Click here to see our selection of Starter Greenhouses.
Grower greenhouses – These are larger greenhouses, often with adjustable shelving, for growing crops full-term indoors under shelter. Diffused or semi-diffused coverings are common for these larger greenhouses. These are all-purpose greenhouses, good for propagating seeds, great for indoor growing of crops, and spacious enough for curing harvested crops and preparing them for storage. Click here to see our Grower Greenhouses.
Another way of differentiating the types of greenhouse is by temperature level. Whichever type of greenhouse you purchase, you have the choice of three basic ways of approaching greenhouse gardening – hot, warm and cool.
Hot house greenhouse – or 'hot house', has a stable temperature in the range of 65 – 70 degrees, or higher. This temperature range is used to grow exotic and tropical plants. Heaters and grow lights are usually needed to maintain this level of heat.
Warm greenhouse – has a stable temperature in the range of 50 – 55 degrees. In this temperature range, you can grow plants which you would grow in your outdoor flower or vegetable garden. You may need to use grow lights or heat lights to meet the heating needs during the winter night months.
Cool greenhouse – or 'frost-free' greenhouse, has a stable temperature in the range of 40 – 45 degrees. This is an ideal temperature range for a 'nursery' greenhouse, where you will be germinating seeds and raising your own starter plants and vegetables for the summer months. You generally don't use grow lights or heat lights in this environment.
What factors should be considered when looking to buy a greenhouse?
Overheating is the principal cause of plant failure in greenhouses. A greenhouse must retain warm air during the cooler months, but also needs to release hot air during the warm months. When choosing a greenhouse, give consideration to the airflow. There should be sufficient allowance for air to enter and exit the structure. Look for vents near the top of the structure and base vents for air intake.
In addition to manually operated vents, solar powered louvers are available which will ensure that vents open when needed. This lets you be away from the garden during the hottest parts of the day without having to worry about your plants being overheated. Some greenhouse designs also feature exhaust fans to prevent overheating during spring and summer months.
The prevailing weather in your gardening region will dictate the amount of insulation required for your greenhouse. The greenhouse covering should hold heat in your greenhouse, naturally keeping it warm when cold weather hits and allowing you to heat your greenhouse inexpensively.
Greenhouse coverings offer varying degrees of insulation based on their inherent properties. Here is a chart showing the relative differences in insulating value for greenhouse coverings.
Beyond the insulation value of the covering material, greenhouse suppliers offer a variety of insulating schemes, depending on the needs of your growing environment. Foundations, walls and roofs can be insulated. For the do-it-yourself gardener, an inexpensive method is to use bubble wrap. Some garden centers provide UV-stabilized polyethylene bubble wrap for this purpose. We recommend that you choose wrap with large bubbles as this has better insulating properties and lets in more light.
Panel clarity: diffused (opaque) vs. clear
Some greenhouses are designed with clear coverings or panels, while other models have translucent or opaque coverings. And some greenhouse models have semi-diffused covers which provide some of the benefits of both clear and diffused models.
Clear panels deliver direct light while opaque panels provide diffused light. If you want to buy a greenhouse to germinate seeds and grow starters which will be transplanted outdoors, then a clear covering has the advantage of bringing full, direct light to the starter trays. This warms the soil and encourages germinating seeds to sprout and develop into a vigorous starter plants for transplanting. Examples of clear-covered greenhouses are the Nature, Clear View and Snap and Grow greenhouse models.
If you plan on growing the plants to maturity in your greenhouse, a diffused covering has the advantage of providing even light for balanced foliage growth as well as preventing hot spots within the greenhouse.
While it may seem counterintuitive, diffused light provides superior light for growing plants. Diffused light may appear to be less bright than direct light, but the light that generally produces the most efficient photosynthesis is not visible to the human eye. Diffused light reaches your plants from many angles, so plants do not get ‘leggy’ as they would when competing with each other to grow towards available light. With diffused lighting plants develop a more balanced, compact structure. The Solexx greenhouses all have diffused covers.
For gardeners who want a dual-use greenhouse, the semi-diffused cover provides the benefits of both clear and diffused covers. The semi-diffused covers allow enough "semi-direct" light for seed propagation in early spring, and once the starters have been set out, the greenhouse can then be used to grow full-term crops inside. The semi-diffused covers provide an all-purpose greenhouse. The RIGA line of greenhouses all have semi-diffused covers, as do the Riverstone Monticello and the EasyGrow model.
As a general rule, clear greenhouses provide warmer, direct light which benefits seed propagation. Greenhouses with diffused coverings are ideal for growing crops full-term. Greenhouses with semi-diffused covers provide the benefits of both. Some greenhouse designs have diffused covering on the roof and clear covering on the sides, which offers some benefits of each.
Sunlight breaks down plastic over time. For maximum life expectancy, greenhouse framework and glazing should be UV-treated. Look for greenhouses that are UV-certified. Some greenhouse coverings have a UV coating on the outside and you have to be careful not to damage the outer coating.
The covering material of your greenhouse needs to be strong enough to resist breakage from hazards associated with your growing region. If there are trees nearby, the covering should resist falling branches. It should be impervious to hail and have sufficient impact strength. If you live in an area with winter snow or icing, the greenhouse covering must be strong enough to support the extra weight.
You should be able to configure the shelving to suit your greenhouse gardening plan. Shallow shelves are ideal for germinating seeds and establishing starter plants, but taller shelf spacing is needed for mature plants. Some greenhouses are designed to accommodate extension kits which give you additional more growing space. What materials should prospective greenhouse buyers look for, or avoid?
Materials used for the glazing, or outer covering, of greenhouses have come a long way since the old-fashioned glass greenhouses. Different types of plastic, and tempered glass, are used today for greenhouse applications. These come in the form of sheets or panels, and range in clarity from clear to diffused.
Glass and clear plastic function in basically the same way, by allowing light to pass through while trapping heat inside. Plastics are generally lighter weight and less expensive, but not as durable to weather and age. Glass greenhouses cost more to build initially, but hold up better over time.
When choosing which greenhouse to buy, you’ll need to consider whether you want it for a ‘season extender’ or if you intend to grow vegetables indoors through the winter. If your region has cold winters, i.e. if it freezes or snows, a well-insulated covering is needed.
~ Tempered glass – Tempered glass panes are very strong and impact-resistant, and will withstand expansion and contraction during seasonal temperature changes. Single-pane thickness of 3mm is adequate for greenhouses, but 4mm thickness is stronger and provides additional insulation value. The edges must be protected during installation, as they will shatter the pane if hit with a hard object. Tempered glass is more expensive than polycarbonate panels, but it is more durable and scratch-resistant. Tempered glass is clear, and provides no diffusion.
~ Fiberglass - Fiberglass is translucent and provides well-diffused light. Fiberglass retains heat more efficiently than glass. Fiberglass greenhouses are usually corrugated to provide rigidity. The outer gel coat will become sun-baked, often within 6 – 10 years depending on exposure. The surface eventually becomes etched and yellowed, and dirt and debris can collect in the valleys.
~ Polyethylene film – UV-treated polyethylene film is commonly used to cover large commercial greenhouses because it is relatively inexpensive and easy to maintain. It is translucent, which provides a semi-diffused light, and retains heat well, especially when double-walled. Polyethylene films only last 3 – 5 years in outdoor conditions, and are subject to stretching and sagging in windy locations or after being loaded with snow. This material can be poked through by falling branches, or torn during winds if not well secured.
~ Twin-walled polyethylene – Twin-wall, high-density polyethylene sheets are more flexible than polycarbonate panels, more rigid than polyethylene film, and come in rolls or sheets as covers for greenhouses. The material is 2-ply, with inner walls which separate the sheets and create an internal air space. This provides excellent insulation value. Twin-wall polyethylene is infused with UV inhibitors and provides soft, diffused light while letting about 75% of natural light to pass through. Twin-wall polyethylene is milky white, or opaque, in appearance. An example of twin-walled polyethylene is the Solexx line of greenhouses.
~ Polycarbonate – Durable and lightweight, UV-treated polycarbonate is a modern, high-quality covering material for greenhouses. Available in different thicknesses, polycarbonate panels provide the clarity of glass but are not as strong or scratch resistant as tempered glass panels. Single-walled polycarbonate lacks the heat retention and strength of multi-walled polycarbonates, and single-wall polycarbonate provides no light diffusion. Polycarbonate coverings have a relatively long lifespan of 15 years or longer in most regions.
~ Twin-walled polycarbonate – Multiple-walled polycarbonate greenhouse coverings are very popular because they have internal air spaces which provide added strength and excellent insulating values. Of particular importance, multi-walled polycarbonates provide diffused light.
~ Triple-walled polycarbonate – This covering has all the benefits of twin-walled polycarbonate, but superior strength and heat retention qualities. It is especially useful in cold climates for year-round indoor gardening. Triple-wall polycarbonate will withstand heavy snow loads and freezing without cracking or becoming distorted.
A greenhouse frame provides structural integrity and serves to anchor the greenhouse covering. Greenhouse frames are available in a variety of materials.
~ Aluminum – Aluminum provides a strong, lightweight frame that will not rust. It is the most widely used material for greenhouse frames, and it has a very long life span. Aluminum frames commonly have extruded channels which provide a perfect fit for inserted covering panels. Aluminum is not an insulative material, so there will be some heat loss through the framing. Powder-coated aluminum retains a smooth outer finish as well as providing different colors.
~ Steel – Galvanized steel frames, usually tubular, are extremely strong, long lasting and low cost. Because of steel's inherent strength, minimal structure is required for framing, which results in additional light getting through to the plants. Polyethylene film is often used for covering steel framing because other covering materials require fastening systems which do not lend themselves to steel frameworks. Steel frames are heavy, which helps the greenhouse remain stable in windy conditions, but the weight makes transporting and assembling the greenhouse more difficult. Steel is usually used for large commercial greenhouses.
~ Plastic resin – Plastic frames have become popular because they are attractive, less expensive than aluminum and do not conduct heat out of the greenhouse as much as steel or aluminum. Plastic frames lack the strength of metal frames, so they are used primarily for smaller greenhouses which have shorter dimensions. Plastic frames are often designed to be used with polycarbonate panels.
~ Wood – Wood greenhouse frames are often used in do-it-yourself greenhouse building projects. Wood can provide an aesthetically pleasing appearance and sufficient strength and durability. However, wood is susceptible to rot, so the construction should ensure that the wood does not contact the ground or any moisture holding surfaces.
What are a few important greenhouse accessories to consider?
When you purchase a greenhouse, take into consideration what accessories are included in the purchase price. Although most greenhouses are sold 'ready-to-use', there may be some options you will want to add to help ensure your success with greenhouse gardening. You can also buy a simple starter greenhouse and add these accessories as needed and as your budget allows.
Automatic louvers – Greenhouses usually come with opening vents which are essential to controlling interior heat. Solar-powered automatic louvers are available which enable the gardener to have a life outside the garden. These louvers will open and shut according to preset heating levels, and are a gardener’s best assurance that plants won’t be lost to overheating.
Automatic irrigation systems – We have lost entire sets of seedlings because we forgot to water them in time. Drip-irrigation systems, drip tape (flat hose with tiny holes), and soaker hoses can be set on inexpensive battery-operated timers which you can set onto your faucet bib.
Greenhouse shade cloth – Especially useful for clear or semi-diffused greenhouses, shade cloth can be draped over the greenhouse roof to lower the inside temperature. Look for UV-stabilized knitted shade cloths with brass grommets, available in various densities. A simpler, very inexpensive method is to use split-bamboo blinds, which you can often find in shops. Shade cloths should be secured at the top of the structure and installed so that it is easy to draw them back during morning and evening hours as needed.
Hand-watering wands – It’s easy to flatten young sprouts with too strong a water spray. Watering wands with adjustable nozzles are designed to let you reach to the back of seedling trays and different shelf heights without having to bend over. Fine ‘misting’ settings deliver water to germinating trays and seedlings without disturbing the soil or the tender sprouts.
Potting bench – When growing your own starts from seed, you’ll need a firm surface to work on, a container to hold the potting soil and an easy way to sweep the soil spilled during potting back into the potting soil bag. Potting benches are designed with all this in mind, helping you keep the potting accessories in one place and the overall appearance of the greenhouse more organized.
Galvanized steel braces - are available that will allow you reinforce the base of your greenhouse. This is useful in areas where strong winds may impact your greenhouse and potentially shift it from its foundation.
Custom shelving – Different styles of greenhouse shelving are available which enable you to customise the interior layout of your greenhouse. Aluminum shelves with interchange heights are popular. Others can be re-arranged into different geometrical shapes for attractive displays and maximizing space usage. They come in a variety of colors, with plastic liners, wheels and all sorts of different options.
Heaters – Many different types of greenhouse heaters are available. Grow lights generate considerable heat for the plant, although they do not warm the soil. Some lamps or ‘toaster-coil’ types will spot heat a corner. Others, such as track systems, can be attached to the roof and heat the entire greenhouse in the winter
What is involved in assembling a greenhouse?
Most greenhouses are sold as greenhouse kits, and are shipped flat in multiple boxes. The customer is expected to do the assembly. Before purchase, be sure to check the assembly requirements. Which greenhouse you buy will be partly determined by the assembly requirements. Some greenhouse kits only take 4 – 5 hours to assemble, but it is safe to assume that assembly will be a weekend job for two people. No special tools are needed, but you will need basic handyman skills.
One of the most important aspects of assembly, however, is the site preparation. At the very least, you will need to provide a flat, solid and level ground space. Larger greenhouses require more site preparation since the ground area is larger and some plans call for cement or brick foundation work to be in place prior to assembly.
Some greenhouse manufacturers realize that ease of assembly is an important factor in choosing which greenhouse to buy. The Monticello line of Riverside Greenhouses, for example, has thicker walls and roof panels which require fewer brackets and supports to prevent collapse. The manufacturer uses fewer differentiating pieces, so assembly is faster and less confusing.
While assembly time is important, it should not be the major factor in choosing which greenhouse to buy. Regardless of difficulty in assembly, the time spent in assembly is minor when compared to the many years of use you will enjoy from your new greenhouse.
Tips for the first-time greenhouse purchaser
~ Read the Warranty. The warranty may vary by manufacturer. When deciding which greenhouse to buy, don't forget to check out the warranty. How long are materials guaranteed to last? What is the duration of the warranty? What parts are covered? Your home insurance policy can also be amended to cover your greenhouse from weather events, such as strong winds and hail, which may not be covered by the greenhouse warranty.
~ Minimize shadows. For clear glass and single-pane polycarbonate greenhouses, you may be able to decrease shadows by spreading white gravel on the outside of your greenhouse along the base. The white color reflects light into the greenhouse and will eliminate some of the shadows.
~ Inspect panel fitting system. Some panel-type covering systems use clips to hold the panels in place, while others use rubber seals. The rubber seals provide better insulation.
~ Look for smooth verticals. Are vertical surfaces smooth? Exterior wall coverings on some designs are full (single) panels which provide continuous sheeting from top to bottom. Other designs have smaller panels set into frames which run horizontally. Any protruding horizontal lip or edge will collect dirt or grow moss over time.
~ Up the size. It's a common refrain among greenhouse suppliers – "Buy one size bigger than you planned." We agree. With greenhouses, garden sheds and woodsheds, you invariably find ways to fill the space more than you expected. This is one reason so many greenhouse extension kits are on the market. If you want to stay with the original size planned, you may consider leaving room for an extension when you lay out the greenhouse 'footprint' in your yard.