Gardening with Chip

Planting over septic fields

A review of articles about planting on a septic drain field offer a range of opinions about what, if anything, is suitable. The concerns seem to be focused on three areas: that compaction of the soil over the drain fields by gardening activities may lead to a reduced life for the septic drain field; that root encroachment into drain lines can lead to drain field failure; and the risk that septic effluent might contaminate fruits or vegetables grown over a septic field with human pathogens.

A septic drain line is engineered to function with the natural soil drainage and rainfall to decontaminate the effluent. It is composed of a series of perforated pipes that allow the septic water to drain into the soil. The pipes are installed 18 to 36 inches below the soil surface in a series of lines that may be up to 100 feet out beyond the septic tank to which they are attached. Tanks need to be emptied by your local septic tank cleaning company every three to five years, depending on household use levels.

Adequate aeration is crucial to make the system work. Often, lines are covered with gravel/sand at installation to improve aeration. There is a concern that adding soil (as in raised beds) over the septic lines can upset the oxygen flow into the drain lines.

However, rototilling, livestock grazing, and/or walking or routinely driving or doing anything over the drain field area when the soil is wet will compact it. That will restrict oxygen penetration and lead to a degraded system. While the impact of those activities may not be immediately seen, there is good evidence that it may shorten the life of the field from 40 plus years to something far less. However, data is sparse on this topic.

Other writers take the position that it is possible to grow something besides grass over the septic lines as long as the lines are not compacted by the activity. The most common recommended plants are herbaceous perennial flowers, ornamental grasses, ground covers and very shallow rooted shrubs like lavender, azaleas, rhododendrons and blueberries. Annual mulching to keep the soil surface loose and weeds out should keep the aeration in good condition.

One writer suggested making the surface of the actual drain lines into paths covered with gravel mulch and growing plants in the space in between. I’m not sure that is a good idea.

There is general agreement that deep-rooted trees (or really any trees) should be avoided. Tree roots will invade drain lines and plug them up. Cottonwoods and willows have a bad reputation in this regard, but I can’t imagine any trees passing on the opportunity to seek out the nutrient-rich water that the drain lines provide. Since many of our trees have roots that extend as much as 50 percent beyond the width of the mature crown (due to our clay-rich soils), careful location of landscape trees is very important. There are root barrier fabrics that can be installed to keep tree roots from the lines; but often, those are best put in when the drain field is installed. They also aren’t 100 percent effective.

There also is agreement that if you decide to grow vegetables over the drain line, plant only those whose edible portion is off the ground. There is no evidence of disease transmission from septic effluent through roots and systemically into plants. The concern is that human pathogens in septic effluent might attach to the surface of the edible portion of a plant, especially if that edible portion is in the soil like an onion, carrot, or beet. Leafy greens are at risk from a rain splash of septic contaminated surface water. Safe choices include corn and trellised crops like tomatoes, pole beans, peas and cucumbers, but I do not think growing edible crops over drain fields is a good idea.

A clogged drain line leads to a clogged, nonfunctional septic system which will have to be replaced. If your system is failing (a good sign is a rotten egg smell), it is important to get a new drain line put in as soon as possible.

Lime for lawns and gardens

Limestone is a mineral widely used in farming. Limestone, which is either straight calcium carbonate or a mix of calcium and magnesium carbonate (dolomite), is mined and then ground to a fine consistency. Lime works wonders when tilled into soils. First, it provides calcium and magnesium which are deficient in Columbia and Wahkiakum County soils. Second, as lime reacts with the soil chemistry, it makes the soil less acidic. We say it raises the pH of the soil. Most plants prefer a near neutral pH of 6.5-7. Our soils often test at a pH of 5.5-6. As the soil becomes less acidic, the plant is better able to extract important plant nutrients like phosphorus and potassium.

Most vegetable garden soils can benefit from about 125 pounds of lime per 1,000 square feet of garden once every three years. The same goes for tree fruit soils. Most berries like lime, though at a slightly lower rate. Blueberries, however, prefer very acidic conditions and should not be limed.

Lawns should have about 100 pounds per 1,000 square feet applied as a more finely ground “prilled” product (one trade name is Calpril). Flower and shrub beds will do fine with about 75 pounds per 1,000 square feet; but, again, don’t fertilize acid loving shrubs like rhododendrons and azaleas.

Final notes

Take excess produce to the food bank, senior centers, or community meals programs. Cash donations to buy food for the food banks are also greatly appreciated.

Very helpful information sources are your local Extension offices: Columbia County Extension is 503-397-3462. Wahkiakum County Extension is 360-795-3278. The Extension Service offices offer their programs and materials equally to all people. They will all know where you can get your pressure gauge tested.

Advice on future garden topics is welcome by emailing me at chip.bubl@oregonstate.edu.