It seems like anyone who is growing vegetables is doing it in raised beds. There are several reasons why building a frame and adding soil, rather than digging into the ground, has become popular.
Homegrown New Hampshire host Emma Erler, who's also lead horticulturist with Kirkwood Gardens, says raised garden beds can be more productive because the soil is less compacted, has better drainage, and warms earlier in the spring, allowing plants to start growing earlier in the season.
They can be easier to maintain, particularly for people with limited mobility, because they can be built at any height.
For New England gardeners, another advantage is that they can be built in places where the soil is poor or unsuitable for growing food crops. Older homes may have soil contamination from lead and arsenic lingering in the yard from historic exterior paint and treated lumber, as well as compacted soil near foundations.
Raised beds generally require more frequent irrigation, however, and the cost of construction (or purchase) as well as the initial cost of filling with garden soil may be daunting.
Building your own raised beds
Erler says raised beds are ideally no more than four feet wide, so you can reach the center of the bed without stepping on the garden soil. They can be any shape or length.
A raised bed needs to contain at least six inches of soil. A foot of soil is best if you plan to grow vegetable crops with deep roots systems.
You can construct raised beds from kits or build your own; there are a lot of designs available in books and online.
Non-treated, rot-resistant lumber is considered the best material. That includes cedar, oak, and locust wood.
Pressure-treated lumber is infused with chemical preservatives to resist rot, fungal decay, and termites. Erler says pressure-treated lumber may leach heavy metals into the soil, though she says the new pressure-treated lumber isn’t nearly as toxic as older kinds.
“Bricks, rocks, cinder blocks, and plastic decking material will work just fine too,” says Erler. “If a building material can hold soil and doesn’t contain toxic substances, it’s fair game.”
To fill your raised bed, use a combination of quality garden soil and compost, and aim for no more than 50% organic matter. Compost should make up 10% to 20% of the total soil volume.
You can purchase “garden soil” in bags, but it may be more cost effective to purchase a load of topsoil from a local landscape material supplier.
Raised beds should be situated in a spot that receives at least six hours of direct sun daily — and don’t forget to put it close enough for your hose to reach.
If you have a gardening question for Homegrown NH, email or send a voice memo to HomegrownNH@ NHPR.org.
See you in the garden!
Homegrown New Hampshire is a collaboration between Squam Lakes Natural Science Center and NHPR.
