To produce pea plants that only display the recessive phenotype of a trait, the parent plants must be homozygous recessive for that trait. This means that both copies of the gene responsible for the trait must carry the recessive allele.

For example, if you want to produce pea plants that only display the recessive phenotype for flower color (white), the parent plants must both be homozygous recessive for flower color (ww). When these two plants are crossed, all of the offspring will be heterozygous (Ww) and will display the dominant phenotype (purple flowers).

However, if the two heterozygous plants (Ww) are then self-crossed, 25% of the offspring will be homozygous recessive (ww) and will display the recessive phenotype (white flowers). The other 75% of the offspring will be heterozygous (Ww) and will display the dominant phenotype (purple flowers).

Therefore, to produce pea plants that only display the recessive phenotype of a trait, the parent plants must be homozygous recessive for that trait.