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Plans - Gravel Culture

In my opinion this is the best. You can use it to provide interesting landscaping for your yard and at the same time grow enough of a variety of vegetables to feed your entire family and then some. Once set up, it is virtually effortless.

There are 5 parts of a gravel garden (also called a flood and drain or ebb and flow system) 1) a gravel grow bed, 2) a nutrient reservoir, 3) overflow / drain mechanism, 4) nutrient pumping and distribution system, and 5) a cycle timer to control flooding cycles. While there are many differences in how to accomplish these parts and their functions, all gravel media gardens require variations of each of these.

Overview Gravel Culture - Flood and Drain System

The gravel system is much like growing in soil in that you can plant a variety of plants in the same unit. Plants grow faster, healthier and may be spaced closer together or even inter-cropped. The flood and drain system works as indicated in the illustration below. During each nutrient flooding cycle the roots are saturated with the nutrient solution and the stale air is exhaled. As the nutrient drains into the nutrient reservoir, roots remain damp but not saturated and fresh air is inhaled providing oxygen for the roots. This cycle maximizes both nutrient delivery and aeration to the roots which is the main reason why it works so well. We recommend 2 to 4 flooding cycles each day during daylight hours.




Building a Gravel Culture Garden

There are several alternatives for building your own gravel culture garden. The essential parts and how they function is displayed in the illustration below.



  Grow Bed Container & Nutrient Reservoir

The gravel grow bed holds the gravel (replacement for soil) which contains the roots and supports the plants. It can be as large as you wish. Just be sure you can reach the plants from the outside without stepping into the grow bed yourself. The ideal depth should be 6"to 8" but may be deeper if you have enough gravel to fill it and your reservoir holds enough nutrient solution to fill it. It must be sturdy enough to hold the weight of the gravel, plants and nutrient solution at the same time without the bottom sagging and the sides bulging.

It is a good idea to elevate the grow bed for two reasons. 1) You save a lot of back strain by working a garden that is just below waist level, and 2) you can use gravity flow to return the nutrient to a reservoir that is lower than the grow bed. Support the grow bed independently, on a table, above the reservoir unless the walls of the reservoir are strong enough to hold the weight of the gravel, nutrient and plants. In which case, you can stack them on top of each other.

A nutrient reservoir should be at least as large in volume as the grow bed. I recommend 1 1/2 times as large by volume. This will allow for evaporation and a sufficient surplus so that the pump will not run dry. The configuration and position of the reservoir should be such that you will have access to clean and service it - an opening of about 6" to 8". The reservoir may be deep, so long and you have a pump powerful enough to pump the solution from the bottom of the reservoir to the top of the grow bed.

Creative Sources for Ready-Made Grow Beds & Reservoirs

There are several creative sources for grow bed and nutrient reservoir trays using manufactured components and modifying them (see below) for use as a hydroponic garden. You can save hours off your construction effort. For your convenience we have documented some of the sources with links to web sites for purchasing and more information. Click on our Creative Sources for Grow and Nutrient Trays page for these sources, they're great. To modify them and complete the garden, see Controlling the Nutrient Flooding Cycle below.

Build Your Own Two-box System

If you wish to build a two box system from scratch, here's how.


1) Start by building a wooden box using 3/4" plywood. The dimensions can vary depending on the area you wish to use for the garden. It should be 7" to 8" deep but the width and length may vary. A small garden would be 2' by 6', but I would prefer a more standard 3' by 12'. See illustration.





2) After using wood screws and ordinary wood glue to fasten the corners and bottom, secure the corners using 2 "L" shaped angle braces attached on the exterior of each corner.

3) Using 2" by 2" triangular wood molding, secure the intersection of the bottom with all four sides as shown. Use ordinary wood glue to secure.

NOTE: When the grow bed is full of gravel and nutrient solution it is quite heavy. To prevent the sides from bowing out for long (12') beds, use an "L" shaped angle brace securing the sides to the bottom every 4'.

4) Seal the bottom and sides with either vinyl film or fiberglass as follows:

4-A) Fiberglass. If you are not used to it, fiberglass can be tricky to work with. You may need some more direct instruction from someone familiar with it. a) Interior - Use mat strips in all corners and anywhere else where flex may occur for extra strength. Apply the fiberglass resin to all interior surfaces. b) Exterior - The exterior must also be waterproofed using a thin layer of fiberglass or good quality waterproof paint or leak-proof plastic base sealer.

4-B) Vinyl Film. Purchase enough film to cover the entire interior surface without seams (bottom and sides) with enough extra to drape 2 to 3 inches over the exterior of the box sides. Use 10 to 20 mils thick available at either a pool supply store, a building materials supplier, or a local plastic supplier.

Before applying the vinyl, waterproof the entire box with waterproof paint or a leak-proof plastic base sealer. Lay the vinyl in the bottom and up the sides. Add a little extra in the corners being sure the entire surface is adequately covered. Drape a few inches over all top edges and attach to the box using 1" by 2" wood molding around the entire exterior of the top even with the top of the box, see illustration.

Cap the top edge of the box with the vinyl draped over with a 1" by 2" molding to protect the vinyl edge around the top of the grow bed (not shown on the illustration). This protects the vinyl from sun exposure and normal wear and tear extending its life.


In keeping with the same style as the grow bed, construct another box using the same instruction as for the grow bed, above. the dimensions, materials, etc. are the same with the follow exceptions:

  • Build it 6" to 8" longer to allow for maintenance of the reservoir after the grow bed has been placed on top.
  • Use a wider molding to secure the vinyl. The illustration doesn't show it, but the molding should extend about 1/2" above the top of the box to provide a guide to lock the grow bed in place above the reservoir. For maintenance purposes, the grow bed should be removable from the reservoir so that the reservoir may be cleaned periodically.


Controlling the Nutrient Flooding Cycle

We recommend flooding the grow bed with nutrient 2 to 4 times each day for 1/2 to one hour during each cycle. To effect and control these flooding cycles requires 1) a submersible pump, 2) a delivery system from the reservoir to the grow bed 3) a drain system back to the reservoir, 4) an overflow system to control the level of nutrient in the grow bed, and 5) a timer to control the cycles. Rio Submersible Water Pumps, Intermatic 7 Day Digital Timers, Fill/Drain & Overflow Kits, and Tubing may be purchased from our Component Parts Products Page

Nutrient Pumping and Distribution System

Purchase a good quality, fully submersible water pump, well coated for corrosion resistance. The pump should be rated powerful enough to pump at least 200 gph (gallons per hour) to an elevation of 1 foot. Purchase about 4 feet of flexible black (or opaque) inch plastic tubing, or cut about 4 feet of flexible garden hose to use as a pump hose. Attach one end to the pump by sliding over the output fitting of the pump. If it is too loose, use a hose clamp.

Nutrient Delivery and Drain Combined System.

The easiest method is to use the Fill/Drain fitting which is part of the Fill/Drain & Overflow Kit available for purchase. The fill/drain fitting should be mounted in the bottom of the grow bed, near the service opening edge of the reservoir. It is connected to the pump with 1/2" tubing. When the pump is running the grow bed is filling and when the pump is off it allows the grow bed to empty back through the pump.

To mount the fitting, disassemble the fitting and drill a hole in the bottom of the grow tray to accommodate the fitting above the nut and O ring. Insert the top part of the fitting into the hole and secure with the O ring (seal) and nut. Attach a 1/2" vinyl tube on the bottom of the fitting with the other end attached to the pump in the bottom of the reservoir.

The top screen prevents the gravel from returning through the pump but may get clogged from time to time and may need to be cleaned periodically.

Overflow System Controlling the Nutrient Level

The second part of the Fill/Drain & Overflow Kit is the overflow fitting. It is also mounted in the bottom of the grow bed and controls the maximum level of nutrient in the grow bed.

To mount, disassemble the fitting and drill a hole in the bottom of the grow tray to accommodate the fitting above the O ring. Insert top part into the hole and secure the O ring and the bottom part under the grow tray. Adjust the top part at the base of the screen to the desired nutrient level when the grow tray is filled with nutrient. This should be about 1/2" below the surface level of the media. If the top of the media gets wet algae will form where exposed to direct sunlight.

Alternate Nutrient Delivery, Drain and Overflow Systems

If you do not use the Fill/Drain & Overflow Kit, the vinyl tube may be extended from the pump through the service opening and over the grow tray wall to fill the grow tray from the top, see illustration. If you use this method, you must provide another method to drain the grow tray and control the nutrient level in the grow tray.

Alternate Combined Overflow / Drain System

The purpose of this system, is to control the level of the nutrient solution when the grow bed is being flooded and provide for a drain back into the reservoir after the flood cycle is over. The top level of the nutrient solution when flooding should be about " below the gravel level to avoid the top of the gravel from getting wet. See illustration below.

There are a number of options to control the water level and provide for drainage. This is another opportunity to be creative. I prefer a simple PVC standpipe arrangement as shown in the illustration. A) Use a 1 " diameter PVC pipe for the overflow tube secured in place by an appropriate size PVC pipefitting. B) The pipefitting should be a female type with threads on the other side. Cut a slot in the center of the fitting about 1/8" to 3/16" wide with a saw blade down to the top of the threads. . This slot should be on both sides of the fitting. It provides for drainage.

C) Drill a hole in the bottom of the grow bed about the diameter of the threads on the slotted pipefitting. D) Screw the fitting into the hole or use an O ring and nut on the other side. It does not need to be water tight, but it should be secure enough to support the standpipe in place. See diagram.

E) Insert the standpipe in the slotted fitting in the bottom of the grow bed. It should slide up and down freely as a method of adjusting the rate of drainage if necessary during use. Do not cement it into the fitting. Cut the standpipe just long enough to allow water to drain down into the reservoir over the top leaving about " of gravel over the water lever. Also, leave enough to slide the standpipe into the fitting to adjust the drainage. About a " for drainage should be sufficient. NOTE: Leave a little extra length. It can always be cut later to allow for the weight of the gravel and the water causing a sag in the bottom.



  F) To keep the gravel away from the standpipe / fitting drain system, use a 4" to 5" diameter plastic PVC drainpipe, or better yet, you can get black flexible ABS drainpipe with slots for drainage. If you use this type, you can cut 2" slits every 3" around the bottom which can be spread out and placed on the bottom of the grow bed like feet. This method uses the weight of the gravel around the base to secure the gravel guard in place around the standpipe. If you use solid PVC pipe, you will have to either 1) drill " holes about 1" apart in the pipe to allow for drainage, or 2) saw 1" grooves about the width of a saw blade, in the pipe about 2" apart to allow for drainage. Cut the gravel guard pipe long enough be about 1" to 2" above the level of the gravel and not interfere with plant growth. Also, the drainage should be sufficient based on the size of the pump you will be using to allow full drainage and not allow the water level to extend up higher than the standpipe.

As the nutrient is being pumped into the grow bed during a nutrient cycle, it is also draining through the slot you cut in the base PVC fitting. However, the pump is pumping faster that the drain system can drain, allowing the nutrient to fill the grow bed to the top of the overflow standpipe (about " below the top of the gravel). At that point, the nutrient drains over the





top and back to the reservoir, keeping the nutrient level constant. As noted above, be sure the gravel guard drains faster than the standpipe overflow system.

Once the pump is turned off at the end of the nutrient feeding cycle, the nutrient continues to drain until the grow bed is empty. This should take about an hour to 45 minutes. If it drains too fast so that the pump is unable to pump the nutrient level to the proper level before it turns off, push the standpipe down into the fitting, thus cutting off the nutrient flow.

Cycle Timer to Control the Nutrient Cycle

Set the cycle timer to flood the grow bed 2 to 4 times a day during daylight hours. It should remain flooded (flowing over the top of the standpipe or the overflow fitting) for at about 15 minutes on each cycle before the pump turns off and the bed is allowed to drain completely. Set it for 30 minute pumping cycles. The timer should allow for minimum on/off cycle times of down to 30 minutes and be weather proof for outside use. Plug your pump into it and you are ready to start.

Gravel Garden Set Up

Guide for Measurements A garden that is 3 by 5 (15 square feet) and 7 inches deep will use the following:

  • Gravel grow bed will need about 8 cubic feet (224 liters) of gravel.
  • Reservoir will hold about 60 gal (228 liters) of nutrient solution.

Gravel Grow Media Use either construction grade pea gravel, lava rock, cinder rock, expanded shale, or the best to use is manufactured expanded clay. The size of the pebbles should be reasonably consistent at an average of 3/8" (about 1 centimeter) diameter but not larger than " (1 centimeters) or less that " (6.4 milimeters). Avoid gravel that is high in limestone (calcium carbonate).

Besides the size of the pebble, there are two major concerns:

  1. pH factor the nutrient should be balanced at an average of 6.4 pH (slightly acidic). This can vary by the types of plants you are growing but an average of 6.4 pH is usually adequate. However, some gravel mixtures leach excessive base substances during nutrient cycles which makes it almost impossible to maintain a level acid balance. If that is the case, change gravel mixtures (expanded clay) or seal the gravel with phosphoric acid (see "Everything You Need to Know Growing Vegetables Without Soil".
  2. Impurities Ordinary inorganic dirt is generally not harmful to the plants, except that it may clog the pump and the overflow system. Rinse it out thoroughly before use.

Fill the reservoir with tap water. Balance the pH to the desired level (6.4 pH) and add the dry or liquid nutrient per the instructions.

Normally you will need to add acid to balance the pH since most tap water and gravel are both alkaline. Use 25% to 35% dilute sulfuric acid, phosphoric acid or citric acid. Some retail stores carry products called pH Down, which work well but costs more than the liquid acids mentioned above.

Now you are ready to start planting.

Good luck.

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