Sub-irrigated raised planter

Tool Concept


This concept is meant to provide consistent soil moisture levels to gardens with spotty or insufficient access to water. It take the idea of a sub-irrigated planter and applies it to an entire raised bed.
Documentation Wiki: 

Please note: if this projects gets funded I will also be posting some basic construction documents (drawn plans) and then, after construction I will post an tutorial with pictures and notes.


Material list for one typical 4’x30’ sub-irrigated raised bed (does not include tools):

Material QTY COST PER TOTAL/RAISED BED 4’ length, 2-3/8” dia galvanized metal posts 8 $14.98 each $119.84 2”x12”x10’ scaffolding boards 14 $1.50 linear ft $3,150.00 Oz-Post JAWS Quick Brackets 64 $4.47 3pk $95.36 1” PVC pipe 3’ length $2.68 10' $0.89 Silicone Caulk 1 tube $5.73 tube $5.73 1-1/2” metal electrical pipe 2’-10” length $12.45 10' $4.15 Fluorescent Spray Paint 1 can $5.77 can $5.77 Waterproof membrane (vinyl) 6’x34’ piece $259 piece $259 Gravel 1.5 yards $60 yard $90 6’x32’ piece of landscape fabric 6'x300' roll $149 roll $149 Soil Mix 6.66 yards $80 yard $532.80 VOC Free Lumber Sealant 1 gallon $39.97 gallon $39.97 TOTAL PRICE PER RAISED BED $4,452.51

Instructions for construction of one sub-irrigated raised bed:

i. 2-3/8” round galvanized metal posts for each corner and at 10’ intervals. Suggested minimum 4’ length for 2’ underground and 2’ aboveground. Requires 8 per raised bed. –These would need to be sourced from either Build It Green! NYC or other material supply company. An auger or posthole digger plus a post driver would be useful as well since the Ranch currently does not have any of these tools. (8) 4’long 2-3/8” round galvanized metal posts

ii. 2”x12” lumber for sides of the raised bed. These are typically 10’ in length. We would make a double high wall so we need 14 boards for each bed (two of these would be cut into 4’ lengths). – These can be source for free through the BIG! BLOOMS program. There is a minimal fee for cutting which we could avoid if we had a circular saw handy, although we do not have electricity at Smiling Hogshead Ranch. (14) 2”x12”x10’ scaffolding boards

iii. Hardware; (64) Oz-Post JAWS Quick Brackets (for steel 2-3/8” posts) or strong-tie L-brackets (for 4x4’s) plus 128 screws (assuming none get dropped/lost on the ground) will be needed to hold everything together.

iv. PVC or metal piping will be used to fill and allow water to escape from the sub-irrigated reservoirs. A 1” pipe will be inserted into the high side of the raised planter wall at the 4” from the ground. The first planter will be attached to a water source (our rain catchment system) which will fill the gravel reservoir with water. The second pipe will be inserted at the opposite end of the planter 3-1/2” from the ground. This is the depth at which the soil is in contact with the rock bed reservoir encouraging capillary action of the water to feed plant roots above. Excess water will flow out of the first sub-irrigated raised bed and into the inlet of the second for the next raised bed. In this way, however many beds are built can be linked together to form a continuous water chain. There would need to be 3’ of 1” pipe per planter as well as some silicone caulk to seal the pipe to the vinyl waterproofing membrane. This pipe connection will ideally be at or under the ground level but could potentially become a trip hazard and may break if kicked too much. A simple housing will be needed to be created in order to protect the pipe. This could easily be achieved by placing the pipe in a larger conduit and painting it bright orange or reflective white. Assuming a 2-1/2’ wide path between each planter, materials would require 2’-10” of 1-1/2” conduit piping. One can of fluorescent spray paint should cover the entire job including up to 10 or so planters. A 1-1/2” hole drill will be needed.

v. A thick waterproof membrane on the bottom and wrapping a minimum of 6” up the sides of the raised bed to create the reservoir of water. Wrapping all the way up the side would greatly extend the life of the wood. The size of that would be 4’x30’ plus 24” height on each side equaling 6’x34’ vinyl. This material could be provided by a garden member who has access to discarded vinyl billboard material which we would upcycle for this use. A utility staple gun could be used along with liquid nails to affix the vinyl to the inner walls of the raised bed.

vi. A 4” gravel base laid in the waterproof membrane and formed with ridges. – 120 square feet of bed at a 4” depth equals 40 cubic feet or approximately 1.5 cubic yards of gravel.

vii. Each bed will require a 6’x32’ piece of landscape fabric which will be used to separate the gravel from the soil. – A 6’ x 300’ roll of professional quality, non-woven landscape fabric can be purchased for about $150, this would cover 9 raised beds.

viii. Clean, preferably organic, topsoil/compost mix. Each bed will have a minimum of 18” of garden soil. – 120 square feet with 1-1/2’ of soil is 180 cubic feet or 6-2/3 cubic yards of soil mix. This could be achieved through a variety of mixtures including 6-2/3 yard topsoil –or- 3 yards topsoil + 2 yards compost + 1 yard peat moss + 2/3 yards sand –or- some other combination of soil and amendments. Since we compost on site, we will provide at least two yards of compost, which we currently have curing, likely more depending on when the project is implemented.

ix. A non-toxic, no VOC wood finish and sealant applied to extend the life of the raised bed. (1) 4’x30’ bed = 68 linear feet x 24” h = 136 square feet. Average coverage is 175-300 sq. ft. per gallon of sealant, less for rough surfaces so each planter would require roughly one gallon of sealant plus 2-3 paint brushes.

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