I always put the makas in my garden as I do with the coffee grounds...it can't hurt !
There was a study done in a university in Okinawa about beneficial properties of kavalactones against pests but it may inhibit plant growth too...but since the makas aren't supposed to be very active after a good kava kneading I do not worry much.
"Herbicidal and Fungicidal Activities of Lactones in Kava (Piper methysticum).
J Agric Food Chem. 2006. Department of Bioscicence and Biotechnology, Faculty of Agriculture, University of the Ryukyus, Okinawa, Japan.
This is the first report showing that kava lactones are plant and plant fungus growth inhibitors. Aqueous extract of kava roots showed high allelopathic potential and strongly suppressed germination and growth of lettuce, radish, barnyardgrass, and monochoria. Nine kava lactones were detected using GC-MS including desmethoxyyagonin, kavain, 7,8-dihydrokavain, hydroxykavain, yagonin, 5,6,7,8-tetrahydroxyyagonin, methysticin, dihydromethysticin, and 11-hydroxy-12-methoxydihydrokavain. These six major lactones in kava roots showed great herbicidal and antifungal activities. Growth of lettuce and barnyardgrass were significantly inhibited at 1-10 ppm, and four plant fungi including Colletotrichum gloeosporides, Fusarium solani, Fusarium oxysporum, and Trichoderma viride were significantly inhibited at 10-50 ppm. The findings of this study suggest that kava lactones may be useful for the development of bioactive herbicides and fungicides."