Unknown. The plant-name is borrowed from or well possibly cognate to semantically congruent Aramaic רִיתְמָא / רִתְמָא (riṯmā), רֹותְמָא / רֹתְמָא (rōṯmā), Hebrew רֹותֶם / רֹתֶם (rōṯem), the rest is likely denominal.
Practice speaking and memorizing "رتم" and many other words and sentences in Arabic.