If you've ever tried to grow basil here, you know that it isn't fond of our cool, maritime Northwest climate. Plants set out in May often sulk, refuse to grow - and then die. I used to work at a local nursery and watch customers come in week after week to buy basil plants to replace the ones they lost the week before. We recommended cloches and bringing plants in at night until temperatures warmed, but that advice fell on deaf ears. Visions of pesto, bruschetta, and caprese salads got in the way, and plants went into the ground much too early.
All that said, it is possible to grow basil successfully in Seattle. If you get seedlings started indoors in mid- to late April, they will be ready for transplanting in early June when the soil has finally warmed up.
It is easy to do this. All you need is a sunny windowsill, some potting soil, a container for the soil - an empty egg carton will work just fine - and some seed.
The fun part is the seed! You generally always have more varieties to choose from in seeds, for any type of plant, than you will find when you buy seedlings already started. Growers can afford to grow what they believe they can sell, and they aren't too willing to try exotic varieties. You, however, have options.
Take a look at this list of basil varieties from Botanical Interests (a Seattle Garden Ideas affiliate). You can choose from lemon basil, lime basil, purple basil, Thai basil, Italian basil, Greek basil, plus organic and heirloom blends. Imagine the possibilities!
It will take about 4-6 weeks for your seedlings to be ready to plant outside. You will need to harden the seedlings off - meaning that you gradually acclimate them to being outside. One way to do this is to cover them with a floating row cover, like reemay fabric, after you plant them. The fabric will keep the plants from being sunburned during the day and hold heat in overnight. After a few days, you can remove the fabric and the plants should be hardy enough to thrive.
So there you have it - everything you need to know about getting basil to grow in Seattle. Enjoy!