I mention this because if anyone puts a Boom Shroom and a Fiery Boom Shroom they don't count twice. Also EMPs don't attract pixies themselves so they would need to be added and maintained if that life was to be counted. Pixie can't be added to bulbs but could be added to the Boom Shroom so whether that is present throughout or not can vary too.
I don't add Gardening Pixie and the times for my EMPs with the other 5 likes are:
Seed to Young 4hr 20m Young to Mature 9hr 20m Mature to Elder 2d 21hr
Harvests during Mature to Elder are every 7.5hr
Minimum time from Seed to Elder = 3 days 10 hours 40 minutes
Note that if you are adding a pixie time may be a little quicker. Also note that the minimum time requires triggering to take place promptly when they're ready.