Hello,

To determine how far your drain field extends, you can start by checking your property's septic system design plans or contacting your local health department or septic system installer for detailed information. They can provide you with the exact measurements and layout of your drain field.

Regarding planting near the drain field, forsythia might not be the best choice as it has a tap root that can grow several feet deep, potentially interfering with the septic system.

Instead, consider planting shallow-rooted shrubs such as:

Boxwood (Buxus)

Inkberry Holly (Ilex glabra)

Japanese Holly (Ilex crenata)

Azaleas

Hydrangeas

These plants are less likely to cause damage to your septic system and can still provide a beautiful hedge.

Please read this MSUE Article - Extreme care should be taken when considering what to plant over and around your septic system - Bindu Bhakta, Michigan State University Extension - October 31, 2013, at this link:

Extreme care should be taken when considering what to plant over and around your septic system - MSU Extension

I hope this helps!