Melilotus spp / sweetclover

Adjectives: annual, disturbed, drought tolerant, highly visible, invasive, non-native, ruderal, seed bank, weed

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There are two main species of Melilotus in the Valley, and in much of the world… M. alba (white sweetclover) and M. officinalis (yellow sweetclover). There are some of “that kind of botanist” who consider them the same species since everything they do is pretty much the same. And there are others who, because they have been considered separate for more than 200 years, and because they don’t interbreed, maintain that they are separate.

Since they are so similar, they will be discussed here as one. But, for sure, as might be expected. yellow sweetclover has yellow flowers and white sweetclover has white flowers. Go figure. In both cases, plants have a characteristic sweet odor. Again, go figure.

The sweetclovers can be annual or biennial and can grow quite tall, i.e. 4–6 feet. The leaves are alternate on the stem, and like true clovers (Trifolium spp.) are palmately compound with 3 leaflets. They branch prolifically and look ungainly but this doesn’t seem to matter to them… they hang with others of their kind.

The flowers of both species are small, floppy, and tubular at base becoming broader toward outer edges. The inflorescences are racemes and the individual flowers mature first at the bottom and then gradually upward. They bloom beginning in the spring (late June in the Valley) and can continue until killed back by frost.

Sweetclover fruit is smallish, bean-like pods typically containing one seed. Nevertheless, there are lots of flowers over the course of a season, which means an individual plant can produce 35,000 and 100,000 seeds! Seeds can be viable in the seed bank for up to 30 years.

Sweetclover was first recorded in the US in 1647 and has been purposely introduced several times since, the reason being that it is a decent forage crop and provides pollen for honeybees (also introduced). It tolerates cold and drought (having a large, deep taproot), but not standing water. In the Valley, as elsewhere, it is now found on open, disturbed soils,  roadsides, vacant lots, waste places etc.

In addition to its forage usefulness, being a legume, it supports nitrogen fixing bacteria and has been used as a green manure to enrich the soil. It has also been used in phytoremediation – fixing poisoned or degraded soils – especially on soils contaminated with dioxin.