Following up on Alan Haigh's call for more research on this subject, I
have had some experience with this too.
Mine was an attempt to overwinter Rosemary plants (an evergreen). I
planted them atop the septic tank so they could benefit from its heat.
First I covered them with leaves with a tarp on top and all plants rotted.
Next I covered them with pine needles and a tarp; few plants survived while
most rotted. I repeated the pine needle experiment one more year carefully
checking the plants to determine when to pull off the needles. That worked
except that I had to remulch the plants every time a Spring freeze was
predicted. Then I decided it was too much work for too little gain and I
abandoned the project.
Jim Fruth
Brambleberry Fruit Farm
4002 Davis Street
Pequot Lakes, MN 56472
1-218-831-7018 (My Cell)