Vegetable Gardening in Raised Beds

Jane Sommers tends a raised bed garden at the Marion Garden.
Jane Sommers tends a raised bed garden at the Marion Garden.
Choose a site where beds will receive a minimum of six hours of overhead sun daily.
Choose a site where beds will receive a minimum of six hours of overhead sun daily.
Raised beds may be made from a variety of materials. Tires and plastic tanks are not recommended.
Raised beds may be made from a variety of materials. Tires and plastic tanks are not recommended.
Setting out branches at the base retains water and feeds the above-planted garden for years.
Setting out branches at the base retains water and feeds the above-planted garden for years.
One method of trellising is to use a 16’ stock or hog panel, arched over one or two beds.
One method of trellising is to use a 16’ stock or hog panel, arched over one or two beds.
Row cover fabric must stretch across the entire frame. It protects crops from pests, sun, and cold.
Row cover fabric must stretch across the entire frame. It protects crops from pests, sun, and cold.
By Jane Sommers, OSU Extension Master Gardener

It's a great time to start thinking about building raised vegetable garden beds for next year’s crop. This article will give you some ideas for designing and using raised beds.

So why should you consider raised beds? Isn’t it just easier (and possibly cheaper) to till up some yard and plant? Poor or rocky native soil, drainage issues, and rampant gophers can all be mitigated with raised beds. They are easier on the back because they are up off the ground. They also tend to drain faster and warm up earlier in the spring, so you can plant earlier.

In choosing a site for your beds you should consider the following: Vegetable plants need a minimum of six hours of overhead sunlight per day, so be sure to place your beds where they will get enough sun. You can scatter several beds throughout the sunny areas, if you need to. Also, make sure they are easily accessible; leave room to get the wheelbarrow or tractor up to each bed. And put them close to a water source. The materials you choose to build them will determine how level or flat your site needs to be. Avoid placing the beds in low areas, where cold air or water will pool. And put them where they will be quick to access when you are “shopping” for your dinner ingredients.

Raised beds can be any size or shape you wish. You can be as artistic as you want or need to be. Certain building materials will allow for more creative sizes and shapes. A width of about four feet is good for most people to be able to reach the middle of the bed from either side. The depth is also variable, depending on the building material you choose and your needs. A minimum of 12” is acceptable for many vegetables, but some, like broccoli and cabbage, need deeper soil, up to 16”. Deeper beds mean more soil, so don’t go overboard. You can even build them up on legs for easy wheel chair access.

Raised beds can be an investment in time and/or money. You can purchase ready-made beds or kits, or you can build them. There are lots a very cheap options, too. Plan ahead in order to maximize materials, space, and light. If done right, they can pay for themselves with years of quality produce. The more user-friendly and productive the growing experience, the more value you will get from your investment. I call my cinder block raised beds “Vegetable Henge”. The odd configuration allows access to each bed with my tractor, and it is easy to add to it or change it around.

You can use many different materials to construct raised beds, including repurposed items. Be sure to line the bottom with ½” hardware cloth to keep critters from tunneling into your beds. If you raise your beds up onto legs, use a solid bottom with drain holes.

Wood is popular for its aesthetics and ease of construction. This can be dimensional lumber or repurposed logs. Cedar planks are popular because they decompose slowly, but they eventually need to be replaced and can be expensive. Rough cut 2” x 12” cedar planks are often available for raised bed construction. Be sure not to use pressure treated wood. While some chemicals were outlawed from most consumer grade pressure treated lumber (PT) in 2003, it is still not recommended that you grow food in soil in contact with PT.

Concrete and natural stone last forever and allow for limitless designs. Stone absorbs and holds heat, allowing you to get a jump start on early spring planting. Cinder blocks are a great option, as they are economical and do not require mortar. Two courses of blocks will make a 16” deep bed. The holes in cinder blocks are their best feature. Placed facing up they can be used to support row covers and trellises, and you can even plant in them, expanding your growing area. I find carrots, onions, lettuce, and herbs love growing in the little pockets. Concerns about concrete blocks leaching chemicals have not been scientifically supported. If you are blessed with a lot of rocks on your property, you can dry lay or mortar those into a virtually free raised bed.

Straw bales are a popular option, because they are very inexpensive and versatile. They don’t last long, perhaps a couple of seasons, but then they can be repurposed as mulch and eventually compost. Set them up on edge, with the straw pointing up, water them to loosen the center, and fill with compost. Or, use them as walls for larger beds. A couple of caveats, however: There is concern that wheat straw may be sprayed with glyphosate, and grass seed straw may have residues from other herbicides, so be sure to source your bales carefully.

Hugelkultur (“hill culture”) is a method of using wood debris as the base of a hill upon which you plant. It is probably the simplest form of raised bed construction and a great way to utilize branches from trimmed trees. The rotting wood retains water and feeds the above-planted garden for years.

You can use metal siding or galvanized steel stock tanks, or even an old bathtub (drill drain holes if necessary). Or drape weed fabric in a frame made from erosion wattles. Use your imagination!

Avoid using tires and plastic tanks, as there is concern about petroleum byproducts leaching from those.

Now that you have built your raised beds you need to fill them up with something good for the plants to grow in. Soil requirements for raised beds lie somewhere between pots and ground soil. If you build completely enclosed beds, with a solid bottom, you can use good quality commercial potting soil.

If your beds are open on the bottom, they are in contact with the native soil and will benefit from that. Your soil should be well drained and fertile, but not so loose that the water drains away too quickly. You can mix some of your native soil with pumice to improve drainage (especially if you have a lot of clay) and organic material such as rotted bark and compost.

A little clay is a good addition to any soil mix since it helps retain water and nutrients. If you must import 100 percent of the bed’s soil, a good formula is to blend 1/3 peat or rotted bark, 1/3 vermiculite, pumice, or coarse sand, and 1/3 well-rotted compost. You can add another inch or two of compost each year, either as an amendment prior to planting or as mulch during the previous growing season. A cover crop can also provide the new organic material for the following year, and help retain nutrients through the winter.

A great consideration, before adding soil to the bed boxes, is to borrow from Hugelkulture and lay branches on the bottom of the bed before adding soil. The rotting wood will retain moisture and slowly release nutrients.

It’s a good idea to add mycorrhizae the first year to get the soil microbes started, especially if using sterile soil components.

Calculate the volume of soil you need by multiplying the bed’s interior width, length, and depth in feet, then divide by 27 to get the cubic yards required. Add enough soil to mound up slightly, since it will compact a bit over time. Don’t forget to include the volume of the holes if you use cinder blocks.

Consider how you will water your beds. Hand watering is a great way to get you into the garden, but it’s difficult to apply adequate water by hand; it takes a long time to add 1”-1½” of water every week with a hose. Sprinkler irrigation can be difficult to control to just cover the beds, and not the surrounding area, wasting water. Drip irrigation, either with emitters or soaker hoses, can work well to slowly apply water without wasting. An overnight soaking once or twice a week makes most vegetable plants happy, especially tomatoes, which like deep water. You may still have to hand water your seeds and transplants until established, however.

A layer of mulch will slow evaporation and help keep weeds in check.

Now that your beds are built and full of soil, it’s time to think about protecting your happy vegetable plants from harsh elements and pests.

Row Covers are a great tool. They are simply any kind of fabric laid over a framework. A simple and inexpensive framework can be made by placing 10’ sections of ½” PVC pipe in arcs over the beds, about 4’ apart. They can be anchored by slipping them over 3/8” rebar stakes hammered into the ground, or, if using cinder blocks, simply push them into the soil in the holes. They are easy to remove if you need to stand or use machinery in the beds.

PVC row cover clips can be purchased from greenhouse supply stores online. They can be expensive if precut, but you can buy the material in long sections, inexpensively, and cut them yourself with PVC cutters. They make it easy to attach and remove any cover material.

Row cover fabric can be used to protect your crops from pests, sun, and cold. It comes in different weights and is one way to reduce cabbage moth and leaf miner larvae. Make sure you use a piece wide enough to reach all the way across the frame. Other materials can be used, too, and may save you some money. 6 mil. clear plastic from the hardware store can be used to create a greenhouse effect for extending your growing season and protect the beds from excess rain. Strips of burlap or shade cloth can be attached above the beds for mid-day shade over cool season crops or plants that tend to sunburn. If birds or deer are a problem put bird netting over the beds and weight the edges with bamboo poles that can be easily lifted up for access.

Weeding is easy in raised beds because you don’t have to bend down so far. Also, since the beds are somewhat isolated from the surrounding yard, fewer weed seeds seem to get established in them. If you have some weeds, try mulching with a deep layer of bark, grass clippings, or dry leaves, or use weed fabric. Black weed fabric does double duty, warming the soil in your raised beds and giving you a head start on spring planting.

Trellising is also easy with raised beds. One method is to use a 16’ stock or hog panel, arched over one or two beds to make a short tunnel. If you have two beds, 4’ wide and 4’ apart, one panel arching 8’ from one bed to the next will give you trellising for both that is tall enough to walk under. Using cinder blocks gives you a lot of options for supporting trellises in the holes.

Don’t forget to experiment a little and have fun with your raised beds!

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