Plants for
Waterlogged Soil

    Carex - sedges P

Many Carex species are bog plants and so may not take too kindly to the wet / dry nature of waterlogged soils. One that I do know that works is Carex buchananii - red fox sedge. Brown /  orange leaves growing up like a fire-work coming out of the ground to about 18" and then tapering away to an indefinite curly tip. Tolerates the wettest of soils. They work very well contrasted against green leaves, gravel or boulders. Maybe worth trying other varieties.
Buy Carex / red fox sedge | Buy Carex / sedge 2

    Cornus varieties - Dogwoods S / T

One of the best shrubs for waterlogged areas for most gardens. Dogwoods are often grown for their winter stem colour which is red or yellow. There are also varieties with attractive variegated leaves. Unfortunately the best ones for stem colour have ordinary green leaves and the best ones for leaf colour have duller stem colour, so you pays your money and takes your choice. White flowers in spring as a bonus.

Dogwoods tolerate the wettest soils. I've had them survive happily when at planting the hole I had dug  filled completely with water when I turned round to get the plant.

For leaf colour; Cornus alba "Elegantissima", white margined leaves, C. alba "Gouchaultii" pink flushed yellow margined leaves, C. alba "Spaethii", broadly yellow margined leaves.

For stem colour; C. alba "Sibirica", plain green leaves, bright red winter shoots, good autumn leaf colour. C. stolonifera "flaviramea", bright yellow-green winter shoots - these are also two of the toughest dogwoods.

All types best treated as coppice stools for best stem colour and to stop them growing into a small tree. Once established after 1-2 years, cut stems back to within 6" of ground level in Feb - March. In this way lots of new brighter shoots and leaves are produced each year.
Buy Cornus / dogwoods

    Lonicera - Honeysuckle C

I don't know of any really wet-tolerant climbers, but have had honeysuckles survive in some pretty awful situations. If the soil smells - don't plant one. If in doubt then take cuttings and try planting one of these rather than buying a big healthy plant in a 2-3L pot for about a tenner which you may then watch die.
Buy Lonicera / honeysuckle | Lonicera x purpusii Winter Beauty

    Populus - Poplars T

Only for the largest of gardens, these need to be about 40m (130ft) from any buildings to prevent  damage, and they're all large trees anyway. If you've the space P. alba, white poplar 70-130ft high and to 50ft wide, is attractive with white undersides to the leaves. Looks wonderful when the breeze rustles the leaves and animates the whole tree.
Buy Populus / poplars

    Pyracantha - Firethorn S

Good old Pyracantha, one of the most useful shrubs in the garden and can be quite stunning in the autumn when heavily laden with berries.  Withstands a fair amount of wetness, but less than most on this page.
Buy Pyracantha

    Salix - Willows T

Most species enjoy wet conditions, but CAUTION, many of them grow into large trees and are one of the worst culprits for causing housing subsidence through their root spread. Don't plant the large types unless you have a very large garden, and then plant well away (40m, 130ft) from buildings.

All have decorative catkins in the spring and many have leaves lighter in colour underneath that "shimmer" when blown about by the wind.

A Smaller safer version is S. caprea "pendula", Kilmarnock willow, Height and spread about 5-6ft, S. about 2ft high and wide, often grafted onto a stem 4ft high. Note that these are both grafted onto rootstocks which may produce suckers that should be removed otherwise the whole plant will revert to the rootstock variety and outgrow the graft.
Buy Salix / willow

    Tamarix ramosissima - Tamarisk S

Not a likely candidate for wet soils at first glance as it is often grown on well drained sea-side soils. I discovered its wet-tolerance when I planted it by mistake once (i.e. I wouldn't have if I'd known) in a dried out waterlogged soil in the summer. By the next spring whereas some other plants around had died, the Tamarix was doing well. Withstands a fairly high degree of wetness, but don't bother if the planting hole fills with water as you're digging it.

Graceful shrub to small tree, wispy frothy pink flowers produced in summer. Height and spread to 15ft.
Buy Tamarix / tamarisk

    Others

Ones that might well work, but I haven't tried so don't blame me if they die! The problem is that dry period rather than permanent wetness.

Ferns - Several types enjoy damp conditions, prefer shade. P Buy ferns | Buy ferns 2

Roses - Shrub and species types are pretty tough, but don't push it too much. S Buy Roses | Buy roses 2