I would certainly encourage what people have said about deepening the border. If this is your main perennial or mixed border, it is very hard to get excited about and get experience with plants without a significantly deeper border. 6' is probably a minimum in my book, with give you barely enough space for three "rows" of plants (though generally we don't plant in rows...). If you have the courage to delve in, there is nothing wrong with making the border even deeper in some sections. Putting in a small stepping stone path can help with maintenance access where the border gets deep and give it some interest in the winter. A couple of well chosen shrub and clematis will give you a lot more height variation and seasonal interest. Choose from smaller viburnums, elders, mockoranges, roses -- any number of plants. And Clematis on obelisks will weave and knit over adjacent large perennials and shrubs. Others may disagree, but in a cold N temperate climate, I personally don't care for most dwarf conifers planted in a bed mainly devoted to perennials. They are very stiff and static looking. And to me they end up looking very lonely in the winter, don't add much. In a crowded overflowing border with no bare soil (my kind of planting), many conifers will not grow that well and may have too much needle damage from crowding by perennials. Personally, I think deciduous shrubs with multiple seasons of interest are better choices, tolerate crowding better, often grow faster, can be used as clematis scaffolding, and in my hands are just more satisfactory. You won't be able to get more than one or two but they will add a lot. Another thing to do will be to add big, shrubby perennials -- ornamental grasses, persicaria polymorpha, etc.

New Jersey Tea prefers full sun and well drained conditions and little on the dry side. Here are some plants that pair well with it and have similar cultural requirements and aren't too tall. Short Grasses: Side Oats Gramma (Bouteloua curtipendula) , Prairie Dropseed (Sporobolus heterolepsis), Little Bluestem (Schizachyrium scoparium) Perennials: Purple Prairie Clover (Dalia purpurea), Prairie Smoke (Geum triflorum), Butterfly Milkweed, Wild Petunia (Ruellia humilis) If you are in the Chicago/Indiana area there are quite a number of native plant sales through the county and park districts that usually have this one for sale in April & May. The Morton Arboretum usually has one as well.

Thanks for the reply. It's a very large mature mixed border and there's not a lot of space left for new perennials. It's also on a fairly steep slope so I have to be careful to plant things that don't typically flop. I learned through trial and error over the years what grows well with minimal maintenance and looks good throughout the growing season. There is already some early season color from knockout roses, heuchera, variegated iris amd May Night salvia and some sprirea. I use some self seeding annuals, particularly cosmos, to carry color through most of the growing season. I,m looking ahead to a graduation party I'll be hosting in early June next year and had the idea that there may be some earlier blooming annuals that can be sown from seed this fall to add a splash of color at that time.

How about Coralbell - they bloom for some time, or take a look at Gallardia (blanket flower) -- they will give you nice color and bloom as long as you keep cutting them back. I like catmint also - and you can shape them anytime - makes a nice mound and consistent color.