Wiktionary
purple loosestrife
n. A semi-aquatic herbaceous plant, (taxlink Lythrum salicaria species noshow=1), having long spikes of purple flowers, native to Eurasia, considered invasive in North America and New Zealand.
WordNet
purple loosestrife
n. marsh herb with a long spike of purple flowers; originally of Europe but now rampant in eastern United States [syn: spiked loosestrife, Lythrum salicaria]