No, it won't kill the cells unless the extract has some cytotoxic properties.

Try to maintain the sterilization of water to avoid the contamination.

All the best

It is better to dilute with culture medium, because of the osmolarity of the mix. An hyper-osmolar environment will cause cells to suffer.

An hypo-osmolar medium can also make them suffer, but if it is too hypo it can make them burst by swelling.

Marc