Actually, fall is a better time for the planting and establishment of hardy plants than spring -- the soil is warm and remains so longer, root growth tends to be more active in fall and fall rains tend to be present to reduce the need for a lot of supplemental irrigation in hot weather. In general, by the start of the next growing season, the same plants planted in fall will equal or exceed the establishment and growth of those planted the previous spring.

August is not exactly fall and in MO you are still looking a lot of warm temps for the next 6 weeks. But if you pay attention to watering, I don't see any reason why you can't begin to fill in your planting now. Ivy is not frost sensitive and colder weather should not overly affect its establishment. Clean out the weeds as best you can and fill in with ivy starts. Keep well-watered (but not soaked) until fall rains arrive.

In spring you can take cuttings of the ivy (just whack off a few of the longest tendrils) and root them for additional plants to fill in even more.

Here, English ivy grows year round and with such vigor it is considered a major noxious weed. One cannot purchase English ivy for outdoor use locally because of that