Assuming that plant height is controlled by one single gene you would have a recessive and dominant allele.
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.