Wisconsin spring is in full swing in May and change is everywhere – budding, germination, decomposition, emergence, migration, fresh new growth, and warmth. We welcome garden volunteers and summer students joining us this month. Our gardening tasks include planting, trimming, weeding, and making garden observations.

Woodlands and woodland gardens are rich with understory flowers (as well as flowering trees) this month. The buttercup, rose, violet, and lily families are well-represented by many species in the spring flora, blooming in May and June. A few species are even named after the month.

Canada mayflower (Maianthemum canadense) is a diminutive native plant of the lily family. It is about four to six inches tall. Non-flowering plants have one leaf, and flowering ones have two leaves and a stem holding the small flowers. Each flower has four tepals and four stamens and develops a two-seeded fruit. This species thrives in filtered shade and can grow on moist or dry and rocky sites. In places where it has little competition, it can spread by rhizome. Seeds are dispersed by birds and small animals.

Closely related plants in the same genus, Solomon’s plume (Maianthemum racemosum) and starry Solomon’s plume (Maianthemum stellatum), are one to two-and-a-half feet tall. The former has more rounded leaves along the stalk and a branched inflorescence. The latter has narrower leaves and an unbranched inflorescence. Both have white flowers with six narrow tepals, six stamens, and a single pistil that becomes a smooth small berry when fully developed. Both species grow well in semi-shaded sites with medium to dry soils and can spread by rhizome growth.

Another native “species of the month” is mayapple (Podophyllum peltatum) of the barberry family. It grows in light to medium shade, in mesic to dry sites with rich organic soil. Non-flowering individuals have single umbrella-like leaves, while the flowering ones have two large leaves on a branched stem. The large flower is held under the leaves and requires insect pollination (outcrossing) to produce fruit. Spreading by rhizome, this species can form large patches that persist through most of summer.

In contrast, lily-of-the-valley (Convallaria majalis L. var. majalis) (majalis means “in May”) is non-native, ecologically invasive, and toxic. This species spreads by rhizome, forming a solid ground cover on rich, mesic soils. It should be removed from native gardens, restorations, and woodlands. When flowering, each plant has two vertical leaves about nine inches in height, and a stalk of pendulous bell-shaped white flowers.  As is typical for lilies, the flowers have six lobes, six stamens and a three-parted seed capsule. The flowers are heavily scented.

Another non-native annual species that begins blooming in May is disc mayweed (Matricaria discoidea), also known as pineapple weed because of the plant’s scent when crushed. It is a small, non-aggressive annual plant in the sunflower family. The small cone-shaped flower heads have only yellow-green disc flowers. The leaves are highly dissected, giving a feathery appearance. It grows in disturbed areas, even coming up in sidewalk cracks and gravel.

The annual Friends of the Arboretum plant sale will be held May 18 in tents near the Visitor Center. You’ll find native plants for every garden – see you there!

—Susan Carpenter, native plant garden curator