Hi Ken:

My first recommendation is not to put annuals out until the first of June.  It's very difficult to wait, but will save you a lot of heartache and work in the end.  We can have a below freezing night any day of the year here in Central Oregon, but the odds are in your favor after June 1st.  That being said, on frost warning nights, you need to move your annuals into a greenhouse or other protected space that will stay above freezing.  You can get a protective cover, also know as row cover, to protect them if they must stay outside.  It really depends on what and where you have them planted.  Another factor to consider is soil temperature.  The soil temperature should be above 55 degrees before you plant for the season.  I have found that if you wait until the soil warms, the plants will grow as fast (or even surpass) those that were planted earlier in colder soil and air temperature.  I'm going to attach a couple of articles that you may find helpful.

Gardening in Central Oregon's Climate

Protecting Garden and Landscape Plants During Cold Weather

Thank you for your question,