I looked at Lavender and Edelweiss and they both have terrible abilities set:
Lavender: Heals 3x more HP from Healing Panels.;
Increases Attack for 2 party members, oneself included, by 10%.
Increases Solar Drive's effectiveness by 20%.
Edelweiss: Party members Heal 10% HP when encountering a Nest Panel.
Increases Attack by 5% when encountering a nest panel. (Up to 15%).
Adds 50 to the party's Speed.
So I don't think that they're any good.
Dipladenia - you mean immune to speed altering? it may be usefull sometimes when there are a lot of webs on mission, so i shall think about getting her after I level up all my pierce girls.