We are going to be having soil brought into our backyard. We were wondering if cutting the rhododendrons plants down to the existing soil level, then burying them in at least one foot of soil above would stop them from continuing to grow and eventually break through the sod we’re planning on replacing them with?
It is likely that mature (large) rhododendrons will grow again from the buried roots, though this may take two or three years.
Rhododendrons contain toxins which inhibit decomposition by micro-organisms, and it may take up to 10 years for the underground roots to decompose fully.
Research into removing rhododendrons as an invasive woodland species in the UK has found that treating freshly cut stumps with glyphosate is 100% effective. Apply 20% Glyphosate solution to the cut surfaces either with a sprayer (if there is no risk of collateral damage to other plants) or a paint brush. The vegetation should be dry, with no rain imminent, and the optimum time to do this is between October and February.
Note, you need to coat the cut surfaces immediately after cutting. If this was not done when the rhododendrons were first cut down, make new cut surfaces on the stumps.