Knock On Wood

choosing the best type of lumber for greenhouse building

What Wood Should I Use For Greenhouse Construction?

choosing the right materials for greenhouse construction is an important part of the planning process – and the categories of materials can certainly splinter off into even more.

One of these categories is wood, an important part of some greenhouse construction. In addition to the quality and type of materials you use, such as plastic coverings (whether poly or panels), frames, trusses, and other components, having the best wood for greenhouse construction is of utmost importance: whether it’s for baseboards, hip boards, doorways, or other framing elements.

Why is that? Compared to plastic and metal, obviously, wood is a biodegradable material. Certain species of trees provide strong wood and lumber, while others rot and fall apart at a faster rate than others over time, and increase the likelihood (and frequency) that you’ll have to replace them. For that reason, being picky and selective for the best wood for greenhouse construction is wise, and can save you both time and money – and there’s quite a few types of lumber to choose from.

WHAT WOOD TO AVOID

“That’s easy” you might think – just avoid softwoods, and go for hardwoods. It’s an easy notion to have, but actually quite untrue. Some softwoods have desirable virtues that are quite useful to greenhouse construction, as you’ll see quite soon. But it’s not about softness versus hardness – in the end, it’s about rot resistance.

Greenhouses are, ultimately, outdoor structures. As such, the wood in your greenhouse building project is going to be left vulnerable to rain, humidity, and even waterlogging due to irrigation, or possibly moisture and fungal exposure if any soil contacts the wood. Lumber exposed to lots of moisture becomes susceptible to fungi, and can thus rot, all the more reason why rot resistant woods are the types to go for.

THE TYPES THAT STACK UP

So you know that rot resistance is key. Now that you’re in the lumberyard, what types of wood should you pick up for construction? Are some among the list of rot resistant types even better than other types?

The answer to that last question: yes, some types of wood are much more rot resistant than others. The following list numbers among some of the strongest types of wood you can source from your greenhouse building.

Not only will these last longest against the elements and prevent eventual replacement, they will also provide robust structural support far beyond any other lumber out there. However, keep in mind that some of the following lumber types aren’t widely available just anywhere – you might have to look harder for these.

BLACK LOCUST

This tree, fascinatingly found in the family Fabaceae (the same family as beans and legumes), is renowned for its iron strength, hardness, and high resistance to rot. Fortunately it is a source of very sustainable lumber as well, found in many parts of the United States and being quite invasive in nature. As such, thinning out of black locust regularly as a lumber source has an overall positive impact on the environment.

Black locust is commonly used as posts in outdoor fencing. Due to high amounts of flavonoids – antioxidant pigments naturally found in plants – it prevents colonization of fungi that leads the way to rot, according to a Purdue University article. In fact, its natural protection mechanisms are so strong that some black locust posts have been known to last up to a century!

OSAGE ORANGE

Also called the horseapple tree, the osage orange is similar to black locust. It has long been used as fence posts and can last up to 100 years in the ground without rotting, as told through Mother Earth News. It’s also equally famous for its flexibility, being used for making bows for archery. The toughest tool handles out there are another popular candidate for osage orange wood.

This wood is a real treat to find for greenhouse construction. However, it is very rare to find in the 2-by-4 forms that would be ideal, since it is so difficult to find trees that grow perfectly straight.. Regardless, if you get your hands on this lumber, know that it is among the strongest you’ll encounter.

PACIFIC YEW

Though rated as a softwood, this evergreen is shockingly hard and dense, and could be called one of the hardest softwoods out there as described on WoodDatabase.com. What more: it has an excellent reputation for rot resistance and, as an extra, insect resistance as well. Lumber from this tree is a favorite for making canoe paddles and bows for archery. Not only is it hard, but its flexibility is something to admire, too.

While it is still being harvested with no major signs of ecological impact, Pacific yew may soon be rendered to a list of threatened lumber sources. In a matter of years, it could be deemed an unsustainable lumber source – so if you happen to find this lumber as an easy and accessible option for greenhouse construction, by all means, go for it. However, consider transitioning to source of more sustainable wood in time.

SECOND BEST, BUT STILL IDEAL

The above lumber choices are top-notch, but not always accessible. Lucky for you, there are others that are much more widely available, and that have rot resistance and other properties of their own. If you can’t get a hold of these above, keep an eye out for the following types – you’ll be certain to find them at most lumberyards.

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BLACK WALNUT

This tree is a common sight on slopes near riparian areas throughout the central and eastern US. It’s quite hard and dense, with reasonable pliability and enviable rot resistance. The downside: it’s quite expensive. It’s a favorite among fancy woodworkers, carvers, and furniture makers because of its beautiful luster and grain.

The roots of the tree when alive release chemicals into the soil that prevent any living plants from flourishing around its base. This may be what is responsible for its fungal resistance, too. It seems that this tree is full of potent, repelling chemicals – and ones that could work in your favor with greenhouse building projects outdoors.

CEDAR

Whether eastern or western redcedar, these varieties (though not true cedars) are very easy softwoods to come by and notorious for their rot-resisting, insect-repelling qualities. They’re also affordable, beautiful, and aromatic varieties to boot. Western redcedar was classically used as wood for canoes, because it would not rot when exposed to water.

Eastern redcedar, on the other hand, was used for fence posts, bows for archery, and furniture. Today, they are popular options for any outdoor wood project: playground equipment, decks, porches, outdoor furniture, and more. For your greenhouse building, this is a good option as well, though it may not be the strongest. However, you’ll be sure to enjoy its perfumed, aromatic fragrance.

CYPRESS

These stately trees can be found growing in the southern United States, while some species also grow in parts of Europe. These, too, were once used as fence posts, but also as for siding, paneling, and furniture, per the American Hardwood Information Center. Preservative volatile oils (one of them known as cypressene) are what prevent the wood from being affected by insects or becoming vulnerable to rot.

The Wood Database adds that cypress – particularly the bald cypress – is quite sustainable as a wood source, being a species of least concern. It’s moderately expensive and fairly available. Its major downfall: some are quite allergic to the wood, while for others, it might cause minor respiratory irritation.

MAHOGANY (AMERICAN)

The right kind of mahogany is well worth it for greenhouse construction. The trick is finding the right variety, as there is a wide set of different breeds. The one you’ll want to go for is Honduran mahogany, also sometimes called genuine, American, or Brazilian mahogany. This tree grows all the way from South America to southern Mexico, so says Wood Database.

While it is rot- and insect-resistant, it’s not the highest up on that scale, though it certainly makes up for it with beauty. They’re also not the most sustainable source of lumber, with this specific species declining in population. If you go for this type and fall in love with its appearance, make sure that you are in fact investing in genuine mahogany. Only this specific type will provide you with the resistance and strength combined with aesthetic that you’re going for.

PRESSURE-TREATED PINE

If you are not stricken with panic at the thought of chemicals, then pressure-treated pine may be the lumber for you. Without the pressure treatment and added chemicals, pines of many types are insect and rot-resistant, though not amazingly so. It certainly depends on the kind of pine you buy (white, southern yellow, black, jack), so do your research there too.

However, pressure treatment ensures and extends its longevity quite a bit, making it a very widely available and wonderful lumber to use – and it looks good, too. If you’re concerned about chemicals from the treatment (such as copper, chromium, and arsenic) leaching into your soils, Fine Gardening assures that the levels of these chemicals tend to be far below harmful. If the thought of it is still too much, there are plenty other options.

REDWOOD

A coniferous softwood, the mighty redwood ranks up there with similar evergreens such as cedar, cypress, and pine in terms of durability. It’s beautiful, rot resistant, and insect resistant. However, you may have a difficult time finding it anywhere else outside of the Pacific northwest (northern California, Oregon, or Washington) or the west coast in general.

While a great candidate for greenhouse construction, redwood is an endangered group of species, and is considered unsustainable. If you have access to lumber milled from naturally felled redwood, definitely consider it an option. Otherwise, if you want to source a regular type of lumber along its same lines, consider cedars or cypress.

WHITE OAK

More than any other type of oak, white oaks are central to the general lumber trade of the US. They have long been a huge part of construction in America, throughout the Industrial Revolution and all the way back to colonial settlement. White oaks are not a single species, but are rather a designation of oak species: including the true white oak (Quercus alba) but also the bur oak, post oak, swamp white oak, chestnut oak, and many others.

White oaks grow all over the eastern United States and parts of the eastern midwest. An article from Purdue University rates white oaks as very excellent when it comes to rot resistance, and certainly up at the top among moderate kinds. The true white oak, if you can get your hands on it, comes as the highest rated of all in this category. It is also a very sustainable wood source.

WILD (BLACK) CHERRY

In the same price range as oak, black cherry is quite affordable, and has a beautiful appearance. It’s widely available and a very sustainable source of lumber throughout the eastern US and eastern midwest. Since it flourishes in areas that are known to be moist (such as slopes and wetlands), it has developed its own natural rot resistance over time.

Comparable to mahogany, black cherry is considered one of the most gorgeous woods out there, and an affordable option too. It is also very easy to work with because it is so lightweight. In spite of its lightness, however, it’s very durable, hard, and strong.

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WHAT WILL YOU BUILD?

Does your greenhouse building require some wood materials? If so, choose wisely and carefully. The kind of lumber you select can contribute to a lot of factors in your setup: such as how much money you pour into renovation costs, as well as the time and work you put into replacing wooden parts that go bad over time.

Some kinds of these high caliber wood choices will be easy to find in your area, others not so much. Still, you have quite a few choices to choose from before you get started building. Don’t settle for less – make sure you opt for high quality wood, and select the right type!

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Adrian White is an organic farmer of near a decade, and a food, health, and sustainable ag writer of 6 years. Her writing can be found in publications like The Guardian and Civil Eats, and she is also a regular contributor to Rodale's Organic Life and Healthline. She lives in Iowa as co-owner of organic farm Jupiter Ridge Mushrooms and Veg, fueled by her passion as a next-generation farmer.

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Adrian White is an organic farmer of near a decade, and a food, health, and sustainable ag writer of 6 years. Her writing can be found in publications like The Guardian and Civil Eats, and she is also a regular contributor to Rodale's Organic Life and Healthline. She lives in Iowa as co-owner of organic farm Jupiter Ridge Mushrooms and Veg, fueled by her passion as a next-generation farmer.

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