Great plants for greywater
Most fruit trees thrive on greywater, and there are many delicious options! They can tolerate frequent watering and once established they can go long periods with no water.
- Choosing a fruit tree: To start, use root stocks that are resistant to local diseases (ask at your local nursery or Cooperative Extension) and plant trees that are known to grow well in your area. Your tree will also do better if it has good soil; adding compost may be helpful.
- Drainage: Next, consider the drainage of your site. If drainage is poor, you will need to plant the tree on a mound and water it less to prevent diseases like crown rot. When planting trees ensure that the crown of the tree is above the mulch basin to prevent crown rot.
- Salt: Fruit trees are generally salt sensitive and should not be irrigated with water from powdered detergents or other products containing salts. If your greywater source contains lots of salt (dishwasher detergent, for example, is high in salt), add salt-tolerant plants to your landscape, or irrigate frequently with rainwater to flush salts from the soil. Plants that thrive on recycled or reclaimed water (highly treated wastewater) are good choices for high-salt greywater; read more here.
Other perennials that thrive on greywater include edible shrubs and vines such as raspberries, thimbleberries, blackberries and their relatives, currants, gooseberries, filberts, rhubarb, elderberry, passion fruit, kiwi, hops, and grapes. Blueberries love acidic soil so you'd have to be careful if irrigating them with greywater, and use pH neutral soaps or acidic mulch.
Where to irrigate trees
A trees' feeder roots extend well past the dripline (the outer edge of the branches). If the dripline is 10' from the trunk (for example), dig a mulch basin 15' from the trunk. (Do the best you can if a foundation or sidewalk is in the way). Plan the system so you can extend the irrigation zone outwards every few years as the tree grows.
Quantity of water
You can either send a lot of greywater to trees that love water, or spread it out more and water trees that need less. A mature tree can absorb huge amounts of water, but it can also live just fine with little extra watering. For example, a medium sized fruit tree (10 ft. canopy diameter) in coastal California is recommended to receive about 12 gallons per day, or 84 gallons a week, and the same tree inland needs about 20 gallons/day, or 140 gallons a week for optimum production. We've observed several similar sized trees in coastal CA that receive a fraction of that and fruit prolifically! Many established fruit trees are never watered in backyards and survive just fine; with just a little greywater they'll begin to fruit more.
As you can see, irrigating with greywater is not an exact science. We usually split up the flow from one fixture (like a shower) to irrigate 4-6 trees, depending on the amount of water, the size of the trees, and the drainage of the site.
Some fruit trees prefer less frequent, deep waterings, allowing the soil to dry between irrigating. Incorporating different "zones" into the greywater system allows for this. You can alternate between groups of trees every other week. Some trees, however, are fine and happy getting watered all the time. This partly depends on the drainage of your site. A simple home percolation test should be performed to see how well water drains in your site. This will also determine how much you need to spread out the greywater so there is not pooling or runoff.
How to irrigate with greywater
Greywater can either be discharged on the surface of the ground, over mulch, or it can be discharged below the ground. If greywater is discharged directly onto the bare ground it can clog the soil by filling the small air gaps in its structure. It will drain slowly, often causing pooling (something to avoid). Mulch prevents this from happening. Mulch can be any type of wood chips, straw, or bark. The new code in California requires the point of discharge to be under 2" of mulch or other barrier.
- Discharge onto mulch.
- Have an air space between the pipe and the ground. This will prevent roots from growing back into the greywater pipe and clogging it.