For a perfect assembler and at a high coverage, the contiguity of the assembly at a finite read length is limited by repetitive sequences. We study the limit imposed by repeat structures in plants, and contrast it to human, as the read length is increased. We started with assembled contigs from long reads and perform an all-against-all alignment. Non- unique regions of the contigs define repeats. We compare the repetitive sequences in human, a fish and several plants including coffee, grape and maize. We show the tendency of repeats to cluster in several plant genomes. Clustered repeats are especially difficult to assemble from short reads because even when all short reads are identified to be from the same 100 kb region, they are still repetitive in the repeat-cluster.