Medicine-Making by the Moon by Gretchen Lawlor

Waxing Moon: In this period, the life force is increasing and rising
through the plant. This period is excellent for harvesting (except for
the roots). This is also a good time to draw energy of the plant out in
a tincture or oil.

Waning Moon: In this period, the vital forces are going down into the
root. This is a good time for feeding roots, and harvesting roots.

New Moon - First Quarter: The first few days of the New Moon are
considered the best for harvesting roots or anything fully ripe (not
leaves though).

Second Quarter Moon: (7 days after New Moon) Good for harvesting and
reorganizational projects. Optimal time to harvest plants with twigs and
stems you want to use. This is because the plant energy is in movement
throughout plant but not quite concentrated in the extremities yet.

Full Moon - Third Quarter: At the Full Moon and 1.5 days before and
after is a time of peak emotional energy. Medicine-making for profound
magickal intent can be made now. Harvest flowers right after the Full
Moon itself. Do most medicine-making a few days after the Full Moon so
that the moon energy has time to stabilize a bit. The medicine good to
make at this time is: to eliminate, shrink or detoxify.

Fourth Quarter Moon: Exists half way between Full and New Moon. Best
time to eliminate weeds. Plants are more willing to let go in this
quarter. This is a recommended time for harvesting roots.

Source: Llewellyn's Herbal Almanac for the Year 2000