AM2 said: why would she steal them? that's retarded, also Yuuka and Meiling being friends and Meiling with flowers in her hair is awesome.
You recall that flowers is part of the yukkuri's diet, right?
Combine that with yukkuris like marisa actually being retarded and being uncaring about other's belongins and territories and you get yukkuris trying to steal somebody's easy place or eat out somebody's crops and plants, in this case such plants being Yuuka's flowers.
Yuuka and Meiling being friends makes sense. In some of the supplementary material, wasn't it stated that Meiling also took care of the SDM garden?
Sengoku Gensokyo made reference to that; Yuuka's special battle dialogue for Meiling reveals that they have mutual respect for each other as gardeners.