Winter Installation of shallow water plants: a risky proposition
Plants are famous for absorbing carbon dioxide and releasing oxygen during photosynthesis, the process by which they store energy as sugar. But they also consume pure oxygen whenever they use the energy they've stored for something practical, such as repairing tissue and growing roots.
Between photosynthesis and exposure to air, normal plants get plenty of oxygen for their upper parts. They have no way, however, to get this oxygen down to their roots. The roots have to fend for themselves by absorbing oxygen from air pockets in the surrounding soil. Waterlogged soil is notoriously lacking in oxygen, which is why over-watered houseplants routinely die.
So how do the roots of shallow water plants grow in low oxygen, waterlogged soil?
Studies show that wetland plants consume just as much oxygen as houseplants. It turns out that they are MUCH better than houseplants at getting oxygen from their above-water parts down to their roots.
Plants that grow with their leaves above water and their roots in waterlogged soil - like most of the species used in constructed wetlands and pond shelves - have large, air filled tunnels called aerenchyma connecting leaves to roots. These plants create pressure differentials that force oxygen down through these air tunnels. They do this by taking advantage of differences in humidity between the inside and the outside of the plant, heat transfer in the leaves, and wind currents. The roots of shallow water plants can get plenty of oxygen for growth - but ONLY if the plant has enough green, metabolizing, healthy leaves extending above the water.
Unfortunately, virtually all eastern US species that grow with their leaves above water and their roots several inches below the normal waterline lose their leaves in winter. Zizaniopsis miliacea (Giant Cutgrass) is an exception; it keeps its huge leaves all year, but in winter the leaves are tan and dormant - not helpful for sending oxygen to roots. Juncus effusus (Common Rush) remains green all year round, as do most of the wetland Carex (Sedge) species. However these grasslike species can really only tolerate an inch or 2 of standing water, and prefer to be above the waterline at least part of the year.
Plant roots need a LOT of oxygen to establish themselves in the first few weeks after installation. In winter, true shallow water plants lack the green, metabolizing leaves needed to get oxygen to the roots. Thus survival of plants which normally grow in 2 to 6 inches of standing water is notoriously poor following winter installation.