I am moving into a co-op which has asked me to take over care of the tree pits.
Thank you so much for your question.
NYBG has two guides that you may find interesting in your new position of care: Street Trees and Urban Gardening. They do not directly answer the question that you pose, but they may add to your knowledge of planting and caring for plants on the city streets.
I also have a book to suggest to you. Hellstrip Gardening by Evelyn J. Hadden addresses the concerns of planting along busy streets and has numerous plant recommendations. This book is available for use in the NYBG Mertz Library but can not be checked out.
Here are a few suggestions of plants from Hellstrip Gardening with attractive flowers and urban toughness that will grow in what sounds like a semi-shade location in our hardiness zone.
Acanthus spinosus (bear's breaches): glossy green mound of leaves up to 3 feet tall with showy white and rose tubular flowers all summer. Will not tolerate bad drainage. Contain plants in buried pots to keep from spreading.
Allium azureum (blue of heaven): bulb with blue sphere flowers in late spring, growing 2 feet tall. Only viable if you get some morning sun.
Amsonia hubrichtii (bluestar): dense, fine textured foliage growing 2 to 3 feet tall with pale blue spring flowers.
Geranium macrorrhizum or Geranium sanguineum (hardy geranium): very tolerant of street pollution and highly recommended. Makes a semi-evergreen, dense mat of attractive, lobed foliage growing 1 to 2 feet high. blooms in spring into summer and can be cut back for more bloom.
Penstemon pinifolius ( pineleaf penstemon): coniferous-like foliage all year with tubular tomato red flowers in late summer and fall foliage color. Only if shade is light.
Ruellia humilis (wild petunia): fuzzy gray-green leaves on dense foot high stems with light mauve flowers in summer through fall.
Scutellaria suffrutescens (cherry skullcap): makes a 6 to 12 inch carpet of leafy ground cover with bright pink flowers in summer to fall. Is marginally hardy in our area but is apt to fare well unless in a very windy spot.
Symphyotrichum oblongifolium (aromatic aster): finely leafed plant with minty smell that blooms in lavender in late summer and fall. Can be pruned to keep low and thick.
Veronica spicata (speedwell): Low growing narrow leaves create a mound from which 18 inch flower spikes in a range of colors emerge.
Annuals will take less special selection as they will be only temporary residents of your tree pit and will not need long term survival qualities. You will be selecting these based on the specific bloom-time gap that you are trying to fill and should get adequate direction on viable plants from your provider.
You will have to take special care if you are planting bulbs to protect them from the ravenous squirrel population of New York City. The products sold as sprays and granules will wash off quickly and are not effective. Your best bet is to create a chicken wire cage for the bulbs to protect them.
Courtesy of NYBG Plant Information