Arboretum Trompenburg holds national plant collections of conifers, Quercus, Fagus, Rhododendron, Ligustrum, Rodgersia and Hosta.
The Gardens include what is claimed to be the largest public hosta garden in the United States (13,000 plants representing over 700 varieties), as well as Annual and Perennial Gardens, several Children's gardens, an English Knot garden and Formal English Garden, Herb Garden, Cactus Garden, Iowa State and All American Test Gardens, Japanese garden designed by Hoichi Kurisu, and Rose Gardens.
Crocus sieberi, Phyteuma sieberi, Luzula sieberi, Eucalyptus sieberi, Cheilanthes sieberi, Callistemon sieberi, Badula sieberi, and Hosta sieberiana, though these names may also have honored Friedrich Wilhelm Sieber.
The gardens contain the National Plant Collections of Lilac, Deutzia, Hosta and Hemerocallis.
Other items of horticultural interest include: holly, daylily, herb, hosta, and heather gardens, as well as more than a thousand varieties of trees, shrubs and flowers, many labeled.
The arboretum contains hundreds of species of trees, shrubs, and flowers arranged in 19 collections (40 acres) as follows: Butterfly Garden, Children's Garden, Columnar Trees, Conifers, Dwarf Garden, Flowering Trees, Founders Grove, Herbs, Hostas, Large Deciduous Trees, Medium Deciduous Trees, Nut Trees, Perennials, Shade Garden, Shrubs, Trees under Utility Wires, Wetland Trees, Windbreak, and Winter Interest.