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Constructing a Raised Bed Using Timbers

Description/Purpose:

A raised bed can be a very beneficial part of the landscape. Their primary use is to provide structure and create a vertical element in the landscape. The raised bed can be constructed of many different types of material such as brick, stucco, limestone, cement, and landscape timbers.

Raised beds may serve one or a combination of the following functions:

  • Add interest to a flat landscape
  • Separate areas of conflicting uses
  • Create additional soil depth for unfavorable subsurface soil conditions
  • Protect plants from animal damage (pets, mice, rabbits)
Image 1

Some of the benefits of a raised planter are:

  • Warmer soil temperatures sooner in the spring allowing for an earlier planting date and a later harvest date in the fall.

  • Improved access to plants. Weeding and harvesting are easier because the plants are closer to you. This is especially beneficial to persons with limited mobility.
Image 2

For more information on accessible gardening consult Accessible Gardening for Therapeutic Horticulture at http://www.extension.umn.edu/distribution/horticulture/DG6757.html or the Chicago Botanical Garden's Enabling Garden at http://www.chicagobotanic.org

This report gives step-by-step instructions for building a two-foot high 4'x 8' raised bed using landscape timbers.

Amounts, Specifications, and Supplies:

  • 15 - 6" x 6" x 8' pressure-treated and stained landscape timbers
  • 40 - 10" long 3/8" galvanized spikes (10 per course)
  • Concrete sand to level base timbers
  • 3 cu. yds. top soil
  • Plant materials
  • Mulch
Tools and Equipment:
  • Chain saw or 6" radius chop-saw
  • Flat-nosed spade
  • Framing square
  • Carpenter's level
  • Drill and 3/8" bit to drill holes for spikes
  • Sand shovel
  • Sledge hammer for driving spikes
  • Sod tamper for compacting sand/soil
  • Stakes and string
  • Wheelbarrow or skid loader
Step-by-Step Process:
  1. Attach a single piece of string to two stakes. Repeat three times so you have a total of 4 pair of stakes with string attached. Note: you will need 2 - 6'6" pieces and 2 - 10'6" pieces of string.

  2. Measure out length (8'6") and width (4'6") of bed in chosen location using stakes and string. Mark corners (where the strings cross one another) with a stake.

  3. Adjust corners to a 90 degree angle using framing square. You may also square them using the 3-4-5 rule (see Building Overhead Structures, Pergolas, and Arbors for a description of this method at http://www.sustland.umn.edu/implement/overhead_structures.html).

  4. After marking corners, place a stake one foot out from the corner along the intersecting lines as in Fig. 1 (this will allow you to dig out the corners). Pull out corner stakes.

    Figure 1

  5. Dig a trench along the inside of the string approximately 8" wide and 6" deep, placing the soil inside the bed.

  6. Smooth out the bottom of the trench so it is roughly level along the length and width using the flat-nosed spade. If the soil is overly coarse

    Image 3

    remove one or two additional inches and replace with concrete sand. Sand is easier to compact and level.

  7. To prepare the trench for the base timbers, level the bottom of the trench along its length and width. To level the trench, start at one end and place a carpenter's level on a screed board (leveling board) on the bottom of the trench. Keep the bubble in the center as you slide the board along the bottom of the trench. If the bubble is not centered, move, add or remove soil until the trench is level. Work your way around the trench, making sure that the entire trench is level.

  8. Compact the sand or soil evenly throughout the trench using the sod tamper. When soil is high in organic matter, recently excavated or generally unstable, simple footings can be constructed using a 6" diameter post-hole digger. Holes should be dug to a depth that reaches below the unstable soil. Using 2" x 6" lumber a square collar 6" x 6" can be created to cover the hole. The holes can then be filled with a pre-measured concrete and aggregate product on site. These simple footings are usually needed only to support the corners of raised beds. Base timbers are set directly on top of the footings and do not need to be secured to the footing.

    Image 4

  9. Place an 8' timber into the trench so that the top of the timber is level with the soil line. The base timber serves as a footing. Orient timber so that it is parallel and touching the marking string line. Check the levelness of the timber along its length and width (add or remove soil as necessary). If soil is added to the trench, be sure it is adequately compacted. Recheck levelness of the timber.

  10. Measure, mark using the framing square, and cut a 4' section of timber making sure that the cut is at a 90 degree angle. Place timber in trench so that it overlaps the end of the 8' timber as illustrated in Fig. 2. Level timber as described in step 8.

    Figure 2

  11. Place second 8' timber in trench and level. Timber should fit snugly up against the end of the 4' timber. See Fig. 2.

  12. Place second 4' timber in trench and level. The cut end of the timber should fit snugly against the 8' timber.

  13. Begin construction of the first course by placing an 8' timber on top of the 8' base course timber so the end of the first course timber overlaps the "seam" of the base course as illustrated in Fig. 3. The first course timber should be directly above the base course timber so that the sides are flush with one another and there is no setback as illustrated in Fig. 4. A setback is not needed for this project as the construction method used will provide enough strength to prevent the sides from pushing out.

    Figure 3

    Figure 4

  14. Larger raised bed can be supported by # 9 galvanized wire holding corners together and every four feet along the inside of side walls.

    Image 5

  15. Have a helper stand on timber so that it does not move. Drill a 3/8" hole 3" from each end and in the center of the timber (Fig. 5). The drilled hole should go through the first course timber and into the base course timber.

  16. Pound a 10" galvanized spike, using sledge hammer, into each hole until it is flush with the top of the timber. Periodically check to see that the timber has not shifted out of place.

  17. Measure, mark and cut an 8' timber in half making a 90 degree cut. Place on top of 4' base course timber so the cut end fits snugly against the 8' timber.

  18. Repeat steps 14 and 15.

  19. Place, drill and spike remaining two timbers in first course.

  20. Repeat steps 13-18 for remaining two courses making sure to offset spike holes to avoid hitting the spikes in the course below.

  21. Fill completed bed with top soil using a wheelbarrow or skid loader. Overfill the bed approximately 6" by mounding soil in center.

    To calculate how much soil you will need use the following formula:

      Length multiplied by width = square feet multiplied by depth of soil = cubic feet

        Example: 8' x 4' = 32 sq. ft. x 2 cu. ft. = 64 cu. ft.

    To convert cubic feet to cubic yards:

      Cubic feet divided by 27 cubic feet per cubic yard = cubic yards

        Example: 64 cu. ft. divided by 27 cu.ft = 2.3 cu. yds. of soil

        Note: Be sure to purchase additional soil to allow for settling.

  22. Water the soil for a few hours to promote settling.

  23. Allow soil to dry out a bit before planting.

  24. Select plants avoiding marginally hardy plants as they are more susceptible to winter damage and dieback.

  25. Plant, and place organic mulch such as shredded hardwood to a settled depth of 3" to conserve moisture and keep weeds down. You will need approximately four bags of mulch (bag volume 3 cu. ft.).

    To calculate how much mulch you will need use the following formula:

      Length multiplied by width = square feet multiplied by depth of soil = cubic feet

        Example: 8' x 4' = 32 sq. ft. multiplied by .4 cu. ft. (about 5") = 12.8 cu. ft.

    To calculate the number of bags needed, divide by the volume of the bag.

        Example: 12.8 cu. ft. divided by 3 cu. ft. = 4.3 bags of mulch

    To convert cubic feet to cubic yards:

      Cubic feet divided by 27 cubic feet per cubic yard = cubic yards

        Example: 12.8 cu. ft. divided by 27 cu.ft = .47 cu. yds. of mulch
Variations of Raised Beds Using Timbers:

Image 6 Image 7


References: Buehler Enabling Garden, Chicago Botanic Garden. 1000 Lake Cook Road, Glencoe, Illinois, 60022. http://www.chicagobotanic.org. Accessed March 4, 2002.

Carpenter, Scott. 1976. Landscape Construction, Landscape Architecture Foundation. Mclean, VA.

Larson, Jean, Hancheck, Anne, Vollmar, Paula. 1997. "Accessible Gardening for Therapeutic Horticulture," University of Minnesota Extension Service, FO-6757-GO. http://www.extension.umn.edu/distribution/horticulture/DG 6757.html. Accessed March 11, 2002.

Lowe, Duncan. 1991. Growing Alpines. B.T. Batsford Ltd., London.

Maxwell, Tom. L and R Suburban Residential Landscaping. Personal interview.

Tanner, Ogden. 1978. Garden Construction, Time-Life Books. Alexandria, VA.


This implementation report was developed by Eric Baldus, student, University of Minnesota Department of Horticulture Science.

 
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