Answer: In nature, once a gap is opened, something will fill it. On the one had you can take your chances that an existing grass or plant will take the place of the bushes once they are eradicated, or you can introduce something else. It may be that grasses are not the best suited plant for your site, or it may be that they do fill in quickly -- fall and spring are prime times for grass development. Your county extension or an area naturalist might have some suggestions as to what would work best in that particular microclimate and soil type. In any field however you will find an influx of colonizing shrubs and trees that need to be mowed off or brush hogged periodically, especially if the field is not grazed.
Multifloras can be very difficult to control, and applications of an herbicide such as glyphosate may need to be repeated. Make sure you are using the formulation/strength for brush and that you are following the directions carefully. Late summer is a good time to use it because the plants are absorbing reserves for the winter and will translocate it to the roots efficiently at a time when they are less able to put out top growth to overcome the effects. Seedliongs will also continue to appear so it is a matter of vigilance to keep it out of a given area. Your county extension may have some useful suggestions in this area as well, since multiflora is considered a noxious invasive alien plant in PA.
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