Ask Mr. Smarty Plants

Are you talking about late THIS summer and into fall? If so, you might consider plastic shrubs, because we don't know of anything else that would work. In the first place, even in Pennsylvania, we don't recommend planting things in summer. That is the time when existing plants are finishing up their blooming and coming into fruit (seeds), to be deposited into the soil, where they will wait out the winter cold and be ready to germinate in the spring. Or, they are preparing to drop all their leaves, because they are deciduous and that is a way to protect the plant from the cold.

Seriously, if you need to block the view of a retaining wall right away, maybe because of a potential sale of your house, there is no plant that could grow that quickly, especially with low sunlight. Even if you could plant a row of shrubs in front of that wall, it could take them years to grow tall enough and to fill in and screen the view of the wall.If you purchase something big enough to do the job, transplanting it would be a difficult job, and it could well go into transplant shock, and either have its growth retarded for 2 years or more, or simply die. 

We would like to make a suggestion similar to one we made to someone who wanted to know how to plant kudzu to cover an ugly wall while the house was for sale. Of course, we were totally opposed to the propagation of a plant that is non-native and invasive. They were in the wrong climate for kudzu and even if it had been ideal, not even kudzu could have covered that wall in the time frame they had in mind. Our suggestion was to first paint the wall with masonry paint, which might not be practical in a retaining wall, which probably has moisture behind it. To go along with that, we suggested simply plainting a row of small shrubs in front of the wall to distract the eye without trying to get them to grow to completely cover it. And, finally, we said if they didn't want to invest in something they would soon (hopefully) move away from, they could buy some plants of an appropriate size and pot them up in large terra cotta pots. Then, when the house was sold, they  could just put the pots on the moving truck and take them to distract the eye from something else in their new place.