I planted beans last week and sowed zucchini. I soaked the beans in water for a few hours before planting directly in the ground. They just starting coming up yesterday. The zucchini have sprouted and I'll be transplanting them in a few days. I'm in Zone 5a (Montreal) so you should be all right if you decide to do the same now. Is there a reason why you want to start from seed? Getting seedlings at a greenhouse or even Loblaw's or Canadian Tire would really be your best option at this time in the season. I just planted all my tomato, pepper and cucumber seedlings this week.

Ask this question on the New England forum also. Your part of MA has winter moths that can defoliate some kinds of trees more than others. There are a number of folks from zone 5 MA who visit the New England forum regularly and will chip in. I'd include a question about winter moth and other pest susceptibility in your post. There aren't winter moths in my area so I can't help with them, but I know that cherry trees around here are susceptible to a wide variety of pests and tend to be short-lived. There was a similar question there recently that might give you some ideas even though her size range started smaller than yours. You might want to consider whether you will want a mix of trees which will give you a longer period of interest and some protection from losing your entire shelter belt if there are disease or insect issues vs. having a more uniform look to your row of trees. You are making a good choice not to look at callery pears. They don't do well with the wet snows that often happen in New England. This is from last fall's Thanksgiving storm and the homeowner had to remove about half of 6 trees.

I have read that those All Gold Hakonechloa are more hardy than the variegated yellow ones. They aren't as elegant, though. The variegated ones need some direct sun here in zone 6b, north shore of Boston. I bought a bunch of them three-four years ago and about half lived; I put them in full shade. Full shade is not good for them here, and I have a friend who puts hers in full sun and they are growing faster than mine. I do have one that grows in full shade, sort of; it survives but does not increase in size. I need to move it. The new little All Gold one I just got is vigorously increasing in size in its shady spot; what a difference.

I wrote this out yesterday before I read Yardvaark's response, so some of what I have written may repeat or contradict what he said. Though you may not want to hear this, I have a few concerns, some practical and one aesthetic. I live in an area that looks fairly similar to yours. I don't know how long you have worked on gardening here, so forgive me if I bring up things you have already considered. For the aesthetic, you want to remember that this is the front of your home, and while the flowers pictured will give you summer interest, you will want some evergreens or other plants with winter, spring and fall interest such as interesting branches, colored bark, etc. to help provide structure and form to the bed in the 7 or so months of the year that your flowers aren't blooming. An evergreen groundcover could also work. You can most likely have flowers but you will also want some year-round plants; otherwise you will have an empty bed for about 2/3 of the year, and while some of the time that is under snow, something like 4 months of the year in my garden are both cold and snowless. Plants that need to be dug up like most of the bulbs pictured (not the lilies in the last photo) need to be planted in a separate part of the bed from shrubs or plants like the lilies that are true perennials in zone 5 so you aren't creating root disturbance. Most of the plants you pictured really like all day sun and good soil, and I am not sure that even having taken down the marked trees and improved the soil you will find they grow well due to the conditions. Some of your trees may be species which don't share space well, so I would take good photos of buds and branching to get them IDed if you don't know what they are. That will allow you to judge how likely you are to get a fair amount of roots growing into the bed from nearby trees (whether the bed is raised or left at grade.) I think that while the trees currently may not shade much of the bed once the marked ones are down, the remaining trees will extend their branches to some extent to take advantage of the newly available sun. In a bed alongside a treed area like this, I might focus on plants that I know will do well with a mix of sun and shade and are relatively tolerant of dry conditions and are hardy enough to not need to be dug up annually. They also will look more natural against the tree line to my eye as the transition from a mass of flowers to native woodland might well look quite abrupt. Dogwood trees (Cornus kousa or Cornus florida), mountain laurel (Kalmia latifolia), hardy rhododendrons, silverbell (Halesia), Epimediums (there's a world-class Epimedium nursery in MA) are some flowering plants that do well along wooded edges for me, but none of them will bloom all summer like those pictured above. I think I would put the flowers in your photos up closer to the house, pehaps enlarging the current bed and replacing part of the lawn (which likes similar growing conditions), including something similar to what I mentioned above for winter interest. I think having a bed at least 8' deep and with at least a foot between plants and the house for maintenance would look better. If you have deer, you might want to consider a high fence or a regular spray program to keep them from eating your flowers since many flowers are quite attractive to deer.