Anything you plant in the planters will need to be an annual as perennials will not usually survive winter in planters in Canada. This means you will not have privacy until the annuals grow to 2ft and after the first frost kills them, so privacy only from mid to late June till early September. And you will have the expense of planting them every year. Finally, I think grasses will look weird on the top of a fence, but that is simply a matter of taste.

For a more permanent solution, consider adding a two foot lattice topper to the fence. Or put up 8ft tall trellises along the fence and grow vines up them.

I agree about the trellises there a lot of nice vines that you can get clamatis,climing roses,hydranga,it will take a few years to grow but it will give you privacy and colour.But always monator how much sun and shade you get so you can put the right plant in the right spot.Thats the best advice any new gardener can get.Enjoy your new deck.

A fast-growing vine is Virginia creeper (Engleman's ivy) which turns a brilliant red in autumn, but it does drop it's leaves each winter. An alternative to trellises would be to build a backdrop of framed-in lattice panels. These can be all the same height, or tiered. That way you have privacy all year, and a nice place to grown any vine you want.

Another fast growing vine is Silver Lace Vine. Grows up to 30 feet a year.
You could always put trellises around your deck and grow something against that.

May I suggest a different approach? I like the idea of privacy without feeling boxed in. Try setting up container gardens to help in privacy and to frame your view.

You could keep a row of potted emerald cedars (columnar evergreens) to form a privacy hedge on the deck itself. You can leave it out during winter and unlike decidous plants, these will remain green during winter and continue to provide you with privacy. By the way, I don't think tall grasses look weird on decks. It's been used often. If you are in a shaded location, another container plant to use is hostas and heucheras.

You could build some tall trellis to hang curtains(using outdoor materials) on.

You can put shade sailcloth on crossbeams to further enhance privacy.

Last, grow a 30 ft tree beside the deck for shade and for privacy too.

Ianna: I agree that grasses, hostas etc look good on decks. The original poster was asking about putting planters with grasses on the top of the fence in order to increase privacy by about 2ft.

Hostas, grasses etc on the deck level will not add the privacy he/she requires.

Deck

Just a word of caution. Lots of great ideas from the follow-ups BUT if you are a newbie, be aware that evergreens in pots only do well if they are kept moist until very late in the season when it is very cold. If they don't have sufficient moisture going in to the winter, they will brown and die. It can be a costly mistake. It is a very good idea though. I have seen it work well and they are very pretty in winter.