Assuming that plant height is controlled by one single gene you would have a recessive and dominant allele.
Genotypes
TT (homozygous)
Tt (heterozygous)
tT (heterozygous)
tt (homozygous)
Phenotypes
Tall Plant
Short Plant
We will assume that tall is dominant and short is recessive. Tall plants must have the big T allele. Your genotypes for tall plants would be
Tt - Heterozygous
TT - Homozygous
tT - Heterozygous
If you cross two heterozygous tall plants you get the following
Crossing two homozygous tall plants would give you
Crossing 1 homozygous and one heterozygous plants, both tall would give you
Crossing two homozygous short plants would give you
not sure if that answers your question but I hope it helps.