While the structure and frames of many modern greenhouses are made from aluminium or tough synthetic materials, there is still a great deal to be said for the traditional wooden greenhouse. Building your greenhouse from timber can create a distinctively attractive and rustic structure suitable for practically any traditionally styled garden, and can be tremendously strong and durable when well-built.
However, a wooden greenhouse is only as good as the wood chosen to build it with, and you should take care to choose a wood that can take the various unique stresses that greenhouse structures must endure. Keep the following factors in mind when choosing your greenhouse timber:
Greenhouses are relatively light structures (especially if glazed with perspex or other synthetic glass substitutes), so it can be tempting to choose an inexpensive and relatively weak timber for construction. However, the weaker your wood, the thicker the frames and supports will have to be to compensate for this weakness, and particularly bulky frames can cast large shadows across your plants to inhibit growth. Weak, lightweight wood can also be too flexible to adequately support glazing, particularly in areas prone to high winds, and can provoke swift cracking and breakages.
This is not to say you should choose the heaviest, strongest wood you can find, however, as this can be prohibitively expensive and will necessitate addition supports to prevent your greenhouse simply collapsing in on itself. Instead, choose a wood that balances moderate weight with strength and durability; pine, Douglas-fir, spotted gum and alder are generally excellent choices.
Mould and moisture
As a hot, tight space with limited ventilation, greenhouses, particularly larger structures housing potentially thousands of plants, naturally become extremely humid. This is good for the plants, but it also benefits less welcome growth in your greenhouse; mould and fungal colonies can thrive in a hot, damp greenhouse, and without proper anti-mould measures many timbers can quickly fall victim to fungal decay.
Choosing a naturally mould-resistant wood is perhaps the easiest way to subvert this problem, and many woods with remarkable resistances to fungal growth are suitable for greenhouse building. Cedars, particularly the famously rot-resistant Western red cedar, are fine choices that are light and strong enough for greenhouses, but may be too expensive for some. Tropical woods such as teak and ipe are also resistant to mould, but can be quite heavy and difficult to purchase from environmentally sound sources. Redwood and black locust are also fine choices, as long as the wood is taken from relatively old trees.
The other way to subvert mould growth is by choosing a treated wood, or treating one yourself. Pressure-treated woods are enormously durable and can be relatively inexpensive, although they tend to have a drab, grey appearance that may require staining or painting to achieve the look you want. Pre-treated plywoods are also suitable, particularly the more expensive tropical and marine varieties. If you'd prefer to treat your wood yourself, pay close attention to the kind of treatment you choose; oil-based treatments look more natural and can bring out attractive shades in your wood, but will require periodic reapplication to be effective. Polymer-based treatments last for much longer without maintenance, but form a waterproof 'skin' on the surface of the wood that can give wood an unattractive 'plastic' appearance.