The parent plants could be either $Tt$ x $Tt$ or $Tt$ x $tt$. The homozygous dominant genotype, $TT$, could not be present in the parent organisms.
Work Step by Step
If we have both heterozygous parents $Tt$ and $Tt$, then the offspring could have genotypes $TT$, $Tt$, and $tt$. This means that the offspring could be tall or short plants. If one parent is heterozygous $Tt$ and the other is homozygous recessive $tt$, then the offspring can have genotypes $Tt$ and $tt$, so the offspring could be tall or short plants. Neither of the parents can be homozygous dominant $TT$ because the offspring would all inherit the dominant allele $T$, so all the offspring would have to be tall, regardless of what the other parent's genotype is.