Landscaping and Plant Selection Tips

Proper plant choice for landscaping in the vicinity of onsite wastewater systems is a concern for many property owners. The key is to select plants that will satisfy landscaping needs while not harming the function of the system. The following information is provided to assist in your selection.

The major risks regarding incorrect landscaping choices involve the following:

• Infiltration of a system by roots effectively blocking pipes, breaking tanks and clogging gravel in leachfields
• The prevention of evapo-transpiration of water from and the transfer of oxygen to a system
• The addition of excess water via irrigation to fields designed to dispose of wastewater

A system may consist of the following units:

Septic tank, sump tank, tightline, sand filter, diversion valve, leachfield(s), mound(s) and sub-drains.

The septic and sump tank are designed to retain water for delivery into the treatment units. The infiltration of roots into these tanks can jeopardize the integrity of the tanks and allow either infiltration of groundwater or exfiltration of effluent directly into the surrounding soil.

Roots may interfere with the proper operation of any of these units. Roots from trees, large shrubs and ice plant have been seen to block 4" pipes and completely infiltrate the gravel of a disposal system, rendering the system useless.

Please use the following information as a guideline for protecting your onsite wastewater system from undue stress:

• Trees or large shrubs should not be planted directly on or near a sand filter, leachfield(s) or mound(s) of either a pressurized or gravity system. Trees that are especially suspect include (Note: This is a partial list):
  • Monterey Pine, Monterey Cypress, Eucalyptus, Willow, Bay, Pepper, Poplar, Alder, Aspen, Mayten, Birch. Staff has also found that both Juniper and Echium roots have caused difficulties for systems.
• Ivy and ice plant retain too much water and restrict evapo-transpiration, transfer of oxygen and the roots clog pipes and gravel. It is recommended that these plants are avoided in landscaping in and around the tanks, sand filter and leachfield.
• Do not cover either your sand filter or leachfield with plastic, as this effectively prevents the transfer of oxygen.
• Sod that requires frequent watering should be avoided because of the excess water added to the sand filter or leachfield area that might result in saturation.
• As a general rule, minimize activities on a sand filter or leachfield area to minimize soil compaction. Never drive across a sand filter or leachfield.
• Remember that generally, there is only about twelve inches (12") of cover soil between grade and gravel and pipes. Plants that have roots that extend beyond twelve inches should be avoided in and around the tanks, sand filter and leachfields. Generally drought-tolerant plants with shallow roots should be acceptable choices.

Recommended Plants

The following plant lists from the County of Sonoma, Public Health Department, Environmental Health Services are recommended for mound or pressurized systems:

Herbaceous Plants

Bulbs

Succulents and Herbs

Many varieties to choose from.

Woody Groundcovers

Content repurposed from Stinson Beach Water District.