Lactuca serriola / prickly lettuce

Adjectives: annual, disturbed, drought tolerant, exposed, invasive, morning, nasty, non-native, weed, white latex

Also known as: milk thistle, compass plant, wild lettuce

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If the first thing you see about this plant is the sweet little yellow flowers, you need to get out more. Before going further, let me note that Earle and Lundin say, quite inappropriately, that prickly lettuce has an “insipid yellow flower” lacking disk flowers. OK, it lacks disk flowers. But insipid? No way. The plant may be ugly as the day is long but the small yellow flowers are cute enough.

Technically, the flowers have a bunch of bisexual ray florets, a bunch being 12-20 or so. Each has a blunt tip with a few (meaning about 5) small teeth. Each ray absorbs moisture, opens, then dries rapidly, meaning they are open only one day, and really only one morning here. The different buds open at different times, so a big plant may have flowers for a month. When there is a mass bloom in a large field, it is simply beautiful.

Beyond that, however, there isn’t much more positive to be said about the plant as a part of the landscape. It is an introduced weed, but what the introducer had in mind, I do not know. It may not be legally “noxious” but it is certainly obnoxious. It is drought tolerant, tolerant of the lousy soils that characterize the Valley (but quite at home in good fertile moist soils if it can get them). It does extremely well in waste places.

But “waste places” are only called that because of human activity. They are the places that we nuked while building housing developments or parking lots. They are the places we dumped the rocks and dirt and then drove away from. Prickly lettuce lives there and in an eon or two, will be replaced by something else after it has modified the soil. It is doing what it can to rehabilitate the ecosystem, I suppose.

Nevertheless, for now, it is ugly. The leaf edges are spiny as is the underside of the leaf midrib (vein). The buds are nothing to praise and neither are the developing fruits (which look like old buds before they open). It has a deep taproot that can develop amazingly fast (but pulls up pretty easily).

But, alas, there are some things about prickly lettuce to admire. For example, like some other annual weeds (e.g. pigweed), it will grow in gravel over weed cloth, or in sidewalk cracks or in rain gutters. It will even flower there, and if it does, it will do it quickly, while the plant is still very small.

Another noteworthy characteristic is the way it holds its leaves. In open sun, the leaves orient in a plane facing east/west such that they avoid the mid-day sun. Because of this, it is sometimes called “compass plant” (although there are other species, unrelated, that have a better claim to that name).

Even in dry conditions, prickly lettuce can transpire rapidly (i.e. lose a lot of water by evaporation through stomates). The result of this is (1) the open stomates make for higher rates of photosynthesis, and (2) the leaf is cooler. Indeed, it is cool to the touch even on dry days.

Prickly lettuce is so keen on reproduction that flower buds, flowers and seed heads are all present on the same plant at the same time.