1) Cornus is usually at least a couple weeks earlier to bloom, when the landscape is still in winter bleakness. This year, however, our one-week transition from snow on the ground to 85 degree sun meant Cornus mas was only a few days earlier than Forsythia, which is just starting to bloom in significant measure as of today (4/25/07, likely peaking within the week).
2) It has a mounding or tree-like form, whereas the Forsythia grows with numerous separate canes. While aesthetic opinions of form will vary, I find the more tree-like form more noble in appearance, and its form is part of the reason that the Cornus needs much less pruning than the Forsythia. Forsythias can be “gracefully arching,” but are large and can also get rather sloppy, tempting owners to top or shear them. (Better for the Forsythia’s health and floriferousness to cut out older canes to the ground each spring.) Cornus mas is a bit bigger than Forsythia, at the largest attaining about 20’x15’, while Forsythias attain about 10’x10’.
3) It is as least as hardy as the Forsythias, many cultivars of which, while being “hardy” in zone 5, will lose flower buds unless they were covered with snow during the winter’s coldest snaps.
4) Cornus mas has “cherry” fruits (½”-long plum-shaped, red). These are not showy as they are hidden among midsummer foliage, but are reportedly very tasty for jelly, or they can be left to feed the birds.
5) Both plants are very flexible as to soil type, and grow well in full sun or light shade. But either will have fewer flowers in shade. (Note that there is another Cornus mas across the street from the library, in a shaded area in the rotary oval, which has about half as many flowers.)
6) Both plants bloom before any significant leaf production. But the Cornus has an airier, more see-through appearance, and would benefit more from a dark background, such as a forest or evergreens, perhaps most ideally a yew hedge. Forsythias also so benefit, but with their generally bolder colors and denser canes, probably need it less.
I have both Cornus mas and Forsythia (cultivar undecided) on my list of plants to obtain for my slowly growing shrubbery. Can anyone in the Berkshires, or in the broader inland Northeast, report on successes or failures, or notable specimens of Cornus mas?