You can plant some things now. Peas and lettuce can go out now. Basil, tomatoes, bush beans and peppers have to wait, probably another month, but you can start them from seed indoors and move them out when it's warmer.

With flowers, it really depends on what you want to plant and whether you're starting from seed or with plants from a nursery.

Most seeds and plants want the soil warm before they go into it. You can put the seed in when the soil is cold, but -if it doesn't rot-- it won't germinate until the temperature is suitable.

link is a list of common vegs and their preferred temperatures. Don't forget that row cover or black plastic is very good at warming the soil beneath.

Here is a link that might be useful: germination temperatures