Question
What do you advise to get luxuriant colour on a trellis in a fairly shady spot? I placed a large pot at the base of the trellis and planted Heavenly Blue, Moon Flower and regular purple and pink morning glories. Only the common morning glories germinated. They  are now blooming and are great if somewhat sparse. The trellis and strings are on an east-facing porch pillar largely shaded by a large maple tree. It gets a blast of direct sun only for an hour in the morning and again in early evening. Next year I want to try for spectacular! Thank you.

Answer
Michael,
This is a tough one... spectacular color on an annual vine in shade is almost impossible.  Do the vines have to be in a pot?  If so, about the only vine I know that grows in shade and flowers prolifically is Dicentra scandens, the yellow bleeding heart vine.  This is really a tedar perennial, but you could grow it as an annual in a large pot - it gets about 10 feet high and blooms yellow all summer (July and August) into fall.  The downside of this plant is that it is hard to grow from seed (when you can find seed) and the plants are expensive.  Wayside gardens catalog usually has them.

You could also try a honeysuckle - all the white or pale pink varieties (NOT Goldflame, a commonly sold pink) do well in shade.  It is a pretty tough perennial so may live from year to year in a large pot.   

Is there room for three pots under the trellis?  You could grow the honeysuckle in one, the morning glories in the other two on either side, and throw in some sweetpeas to boot.  The British pack their vines two to four varieties on a single trellis.  

Another thing to combine with other vines is the Sweet Potato vines - the purple leaf variety does very well in shade, and you can twine it up in and around another plant.  (This vine does not climb on it;s own.)

Another thing I would try is planting Impatiens all around the edge of the vine pots.  Packing them three inches apart, and fertilizing every week, you will have a mass of color below, no matter how the vines do!

I hope this helps...  
C.L. Fornari