Peruvian Primrose Willow

Ludwigia peruviana

Onagraceae

Posting on Thursday instead of the usual Friday, because I’ll be camping in Michigan tomorrow,as primitive as can be after a trip to my favorite restaurant Metzgers in Ann Arbor.  Today’s featured plant is a super-weed.  Everyone has seen this invasive tramp lifting bright yellow flowers out of drainage ditches along the roads.   You’ll see one tomorrow if you drive at all.  Native to Tropical America, Peruvian Primrose Willow has spread across the tropical and subtropical world from West Palm Beach to Polynesia.   It is a weed on steroids capable of rising to 12 feet tall, and its super weed power is what makes this species interesting.  It is Peruvian, I guess, but not a primrose nor a willow.  It is related more to Fuchsia.

Here are some defining attributes of weeds.  With exceptions, weeds:  1.Mostly are sun-loving, and not fussy about soils.  2. Make quickly lots of “cheap” readily distributed seeds with little investment in each individual seed (quantity over quality).  3. Often have delayed germination.  4.  Are often self-pollinated or pollinated by any of many visitors, and/or have asexual reproduction.  5. Frequently have big strong underground parts.

Let’s see, in the case of Peruvian Primrose Willow:

1. Sun-living (yep) and tolerant of every muddy ditch from the Caribbean to Australia.
2. Lots of little cheap seeds readily distributed: The tiny seeds can reach densities of 450,000 seeds per cubic meter downstream from a stand of PPW. They float, blow in the wind, or cling to creatures.   And they can germinate still afloat.    Viability has been reported at 99%.  And if that is not enough, broken fragments of stem can root wherever they wash up.
3. Delayed germination: Some PPW seeds seem to retain dormancy for about two years.
4. Pollination: I do not know if PPW is self-pollinated, although many ludwigias are, so it is likely, especially given the plant’s ability to colonize new places. In any case, floral visitors are many, including bees, butterflies, flies, and probably more.  The flowers have striking UV reflectance patterns.    You and I see them as yellow.  A bee with UV vision sees a target with a bullseye. (The upper left images.)
5. Durable subterranean parts:  That’s the most interesting thing.  There’s a big deep taproot.  Big deep taproots in submerged mud should suffer oxygen starvation.    But our big weed is ready.   At the base of the stem leading down into the root, the cork meristem (the same tissue that makes corks for wine bottles) produces a thick layer of soft spongy white styrofoam to ventilate the submerged portions.

This the the base of the plant split to show the white “cork” ventilation tissue.

Ludwigia peruviana is easy to distinguish from the numerous locally native species, because it is larger than most, with fuzzy parts at least when young, usually has 4 petals and 10 stamens, and a long cylindric fruit.

The plant (by JB)

And now here’s the mystery to “go figure.”   Ludwigia peruviana has 16-ploid strains, that is, with sweet 16 sets of chromosomes.   Although not absolutely connected, self-pollination and extra chromosomes sets often go hand in hand.

And now here’s the mystery to “go figure.”   Ludwigia peruviana has 16-ploid strains, that is, with sweet 16 sets of chromosomes.   Although not absolutely connected, self-pollination and extra chromosomes sets often go hand in hand.