We do not have dilution recommendations and would hesitate to recommend a home remedy like this because you won't know what oil concentration would cause plant damage. (Phytotoxicity is the term for plant tissue damage from chemical exposure, and this can occur when concentrations are too high, when chemicals interact with each other, or when weather conditions make the plant more vulnerable to "burns.") We suggest using registered pesticides (animal repellents are lumped-in with pesticides, which are regulated by the EPA and tested for environmental impacts) if you want to try repelling the deer. Many brands and formulations of deer repellent exist, though we have not trailed them to gauge which might be more effective. (To be fair, none work perfectly since hungry deer can ignore their bad taste and smell, and deer sometimes also get used to certain formulations, so changing them from time to time is advisable anyway.)

Leaf loss this late in the growing season will fortunately not have too severe of an impact on established perennials (like the hostas), though annuals (like the impatiens) can afford to lose less growth since they only live for one year and have fewer root energy reserves to draw upon for regrowth. Breaking the deer's habits of browsing in these areas, though, would still be one benefit of trying a repellent, since once they know something edible is in their home territory, they can keep revisiting that area expecting to find food. Hostas are, unfortunately, very appealing to deer.

A physical barrier like fencing around the yard or around garden beds is a more reliable barrier to browsing than repellents (which must be reapplied throughout the season), but we realize not all yards can accommodate this, and some neighborhoods do not permit fencing. As for how often to apply a repellent, that will depend on the growth rate of the plant (sprays only protect what foliage they contact, so new growth is somewhat unprotected) and how desirable that plant is to deer. Some plants they only like to eat when tender new foliage is emerging in spring, or when flower buds are present, while others might get eaten at any time of year. Different repellent formulations also break down or weather off at different rates, so follow product label directions if they provide a re-treatment interval.

Miri