As in mentally retarded, yes, only something with a brain can be mentally retarded. The GROWTH of plants can be retarded, which means they are not growing as fast as they should.

About 10% of the mentally retarded population is considered moderately retarded.

Approximately 85% of the mentally retarded population is in the mildly retarded category.

The very word Retarded, in one sense meaning slow, explains things. Nearly all retarded people have some form of speech defect, often called Tardive Dysklesia- (fo some reason this is pronounced ( Tardiff). They have an odd nasal speech pattern one might associate with, yes, the Cartoon character Baby Huey- who was supposed to be retarded. Huey finished practically every episode with the sickening boast( I"m a Mental Retard!) Amazing that that was allowed on cartoons in the fifties. They all have a strange, usually slowed drawl way of talking, and it stands out. This should not be confused with a dialect such as Cockney or a Spanish Accent, but is a definite Retarded accent. Colored Girls, for example comes out ( Cold Gales) we had a neighbor who was retarded and from her talk of Cold Gales in Atlantic City, we assumed she was talking about the weather! then she modified it, Cold Gales like the Supremes! Oh boy!

Josh Perry (Ponce), the actor who portrays the Retarded Policeman on YouTube, has Down Syndrome.