If kept too long in a growth chamber after germination, the plugs will stretch and overgrow. Likewise, when the seeds in a plug sheet do not germinate uniformly, the early ones will stretch while the grower awaits the germination of the slower seeds.
Michigan State University researchers have published convincing evidence that in many plants, especially bedding plants, stretching can be reduced by keeping the greenhouse temperatures cool during the day and warm at night. A day temperature cooler by 5° F during production of plugs can result in a higher number of plug sheets displaying near-maximum germination without undesirable overgrowth.
Chemical growth retardants also have proved effective against plug stretching. The retardants are somewhat species-specific though, and should not be used until the grower is certain no damage will result to the plugs being produced.