Cold frame question

If I set up a cold frame, can I grow vegetables and tomatoes year-round?

Depends where you live. Vegetables - probably at least some; tomatos - no.

I am on the Olympic Peninsula in WA state. How do I determine what I can and cannot grow?

Experimentation is one route.

This is a concise and reliable source:

Seattle Tilth's "Maritime Northwest Garden Guide"
Our best-selling book, this is an essential month-by-month manual tailored to our climate and growing season and useful for beginning to advanced gardeners. The 78-page guide outlines each month's garden tasks and lists hundreds of vegetable, herb and flower varieties to plant. It also includes strategies for year-round gardening, articles about organic gardening techniques and activities for kids. $14.95 + tax and shipping.

http://seattletilth.org/get-involved/buystuff

I agree...the Seattle Tilth's book is a great guide for gardening in the region.
When I moved here from SoCal, I was lost in the garden until I got that book.

Another good source is Steve Solomon's "Vegetable Gardening West of the Cascades".

Toms will likely not survive temps under 40f.

Also depends on where you are on the Olympic Peninsula. You have areas that get the most precipitation of any place in the continental US, and places that get less than 20 inches per year. Also depends on fog patterns.

The common wisdom on the wet side of the PNW is that anyplace much north of Portland/Vancouver, you will need season extension to grow a decent crop of tomatoes in the summer. It just doesn't get hot enough for long enough.

Most of the popular gardening books assume that the limiting factor on growing things is cold. That is not so in the maritime PNW - the limiting factors are lack of summer rainfall, lack of heat, and lack of winter sunshine. So growing methods that work well in Pennsylvania or Colorado may not work well here, even though it's warmer.

A very good, recent book is Backyard Bounty by Linda Gilkeson. She gardens a bit north of you, in BC, and has lots of relevant, practical advice to offer.

You can grow almost anything here, but the mechanics are just a bit different than is true for much of the US.

my experience with cold frames is difficulty in watering them, and you have to be able to get the pollinators in for fruit bearing plants..otherwise they are great..

you do have to fuss with venting them unless you get automatically vented ones (i have two with automatic vents, a real work saver)..I used soaker hoses in the cold frames and in my greenhouse to save me hauling water...but in winter they can freeze up..

right now i have tomato chard spinach mustard and lettuces growing in my greenhouse and it has been in the upper 20's for 3 weeks here.