The next Pokemon GO Event is set to kick off today - being announced just yesterday - and it focuses primarily around Grass Type Pokemon. During this short event that only lasts for the weekend of May 5th through May 8th, all Grass Type Pokemon will have an increased chance of appearing and Lure Modules will last for six hours each.
This includes the likes of bulbasaur, Ivysaur, Venusaur, Oddish, Gloom, Vileplume, Paras, Parasect, Bellsprout, Weepinbell, Victreebel, Exeggcute, Exeggutor, Tangela, Chikorita, Bayleef, Meganium, Bellossom, Hoppip, Skiploom, Jumpluff, Sunkern, and Sunflora. However, First Stage Grass Evolutions are expected to spawn much more often than that of Fully Evolved Grass Types or ones that require a Special Evolution Item in order to evolve (Bellossom and Sunflora).
This event is very similar to the Water Festival Event in a number of different ways, being almost identical to it except for the fact that it is much shorter in duration than it and this one is revolved around Grass Types instead. But because of these similarities, many Pokemon GO players believe there may be a small chance that another Pokemon will be able to be found in its Shiny Form (like Magikarp beginning in the Water Event).
Whether or not another shiny Pokemon will be able to be found, Pokemon GO players alike should take advantage of this increased spawn Rate for grass types, especially if they need a number of these evolutions for their pokedex Entries!