While designing and building a raised bed garden, you have plenty of choices when deciding what materials to use to contain the soil. Each option has its pros and cons, and what’s best for another gardener may not be what’s best for you. In this video, I explain the important things to consider and give you the most common options available.
Untreated Wood for Building a Raised Bed Garden
Raised bed gardens are most often used for food crops, and there’s a good chance the plant roots and foliage will come in direct contact with the bed material. For that reason, untreated wood is the best option for most raised beds.
Some naturally rot-resistant untreated woods are walnut, cypress cedar, redwood, oak, black locust, hemlock and osage orange. Even under moist conditions, these types of lumber can last for many years. (If you do choose one of these types of wood mentioned above, check that it comes from a sustainable source before purchasing.)
Keep in mind, all types of untreated wood will need to be replaced at some point.
Treated Wood for Building a Raised Bed Garden
For an inexpensive wood option that will last for years, many gardeners use treated lumber that has been infused with chemicals to preserve the wood. The main concern with treated wood is that those chemicals may leach out of the wood and into the soil, possibly affecting the plants growing in it.
Old treated wood may include dangerous chemicals such as arsenic. You may have heard of CCA lumber. The “CCA” stands for chromated copper arsenate, a pesticide that prevents fungus and stops termites and other pests from destroying wood. Because of concerns about hazards to human health, the Environmental Protection Agency banned CCA timber in residential use in 2003.
Treated wood on the market today has higher concentrations of copper, but no arsenic.
Concrete Blocks for Building a Raised Bed Garden
Concrete blocks, or cinder blocks, are affordable, readily available, and easy to work with — plus, they last a lifetime. The blocks are made from cement and fine aggregates, such as sand and small stones. Some of these blocks contain fly ash, a petroleum byproduct that contains heavy metals, however, these metals will only be released if the blocks break and the dust from the pulverized pieces mixes with soil and comes in contact with plant roots.
Galvanized Metal for Building a Raised Bed Garden
Galvanized metal raised beds are showing up in more gardens today. The galvanization process typically includes dipping metal in molten zinc. Zinc is a micronutrient that plants and humans need in small quantities. If an excessive amount of zinc did leach into soil, it would probably reflect in dying plants before it posed a human health risk.
When using metal raised beds, it’s important to ensure there is adequate drainage and to account for heat. Metal will heat up in the sun and will raise the adjacent soil temperature. Be sure to plant tender vegetables, such as lettuce, toward the center of the beds, where the soil temperatures remain cooler.
Railroad Ties: Not Recommended for Raised Bed Gardens
Railroad ties are a popular raised bed and retaining wall option because they last for many years. However, railroad ties are made from creosote, an oil distilled from coal tar. The EPA says creosote is a probable human carcinogen and notes that it is not approved to treat wood for residential use, “including landscaping timbers or garden borders.”
Tires: Not Recommended for Raised Bed Gardens
Tires are a common raised bed option but generally not recommended. They are a petroleum-based product and will leach chemicals as the rubber ages and breaks down. They just don’t mix with food crops.
Premade Raised Bed Garden Kits
Raised bed garden kits may be the best of all worlds. They are convenient and generally safe.
You can purchase full kits in various sizes that have everything you need, or buy corner brackets that can be assembled with the wood of your choice or composite lumber. The lumber can be cut to any size you wish to achieve the dimensions that best fit your garden.
Raised Bed Liners for Extra Protection
Plastic liners are sometimes used on the inside of raised beds for added protection for leaching and to perhaps extend the life of the bed material. If you decide to use liners, look for food-grade polyethylene plastic. That’s considered one of the safest plastics available.
What is your favorite material for building a raised bed garden? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Three popular courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
*Disclosure: Some product links in this guide are affiliate links, which means we would get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us. None of the items included in this list have any bearing on any compensation being an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, and Wild Alaskan Seafood Box. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.