Answer: Spring is often the wettest season for a swampy area, depending on the cause of the swampiness -- for instance surface run-off, underground springs, seasonal flooding or some other source. Swampy areas are rather delicate in ecological and conservation terms, so I would strongly suggest a professional evaluate it on-site before you begin to interfere with what is already there. In fact, you may find that another area of your yard is better suited to the types of plants you are envisioning growing.
For more detailed information about your specific situation you might also wish to contact your County Extension. In any case they should be able to help you determine if the area is suitable for gardening as well as help you with basic soil tests and interpreting the results with an eye towards planting.
There area number of very cold-hardy plants which will grow in swampy areas including some lovely native and cultivated varieties. Here are a few flowers which come to mind: lobelia cardinalis, marsh marigold, Joe-Pye weed and water iris (Iris pseudacorus). Some trees and shrubs to consider would be the winterberry hollies (Ilex verticillata) the shrub dogwoods, the elderberries (Sambucus canadensis) and of course, willows.
Good luck with your planting!
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