Tubers can be safely planted in the ground after the risk of frost has passed.  For most, this is mid May. 

A good rule of thumb in Western Washington is that if the lilacs are blooming, then it is safe to plant.  Tubers should be planted about 6” deep in well draining soil.  Tubers may take 4-6 weeks to emerge from the soil so do not be tempted to dig them up to see what is happening underground – you will only stunt their growth and set them back. 

If your soil or climate is exceedingly dry then you may need to occasionally water your tubers, but otherwise they should not be watered until you see 3” of growth.  There are many, many resources for growing tips online and I encourage everyone to do their own research.