There's a nursery one town over from here where I buy most of my started plants each spring. The owner knows how to grow seedlings to near perfection so that they are just right for setting out. This same man also produces many that have gone slightly past optimum planting time; seedlings that are somewhat taller than the norm and already in flower. He does this for a very simple reason: They sell far more readily than premium quality seedlings.
The American homeowner, brought up to expect good looks with almost every product he buys, invariably reaches out for the petunias or marigolds that are already in flower, or the tomatoes that have already set fruit. That's a pity. For the most part, European gardeners know better. And so do an increasing number of American gardeners.
Follow one of these more knowledgeable gardeners around a garden center and you will note that he will:
Select stocky, well-branched seedlings that are not yet in flower. If all that is available are plants already in flower, pick off each flower before setting them out. This is important, because maintaining flowers consumes a lot of the plant's energy that would otherwise be used in getting itself quickly established in the new environment.
Preferably choose seedlings growing in cell packs where the flat is divided into separate compartments for each plant. This way there is far less damage to the roots at planting time. If only old-fashioned communal flats are available, take a sharp knife and cut down between the rows of plants so that each plant has its own block of soil. By doing this four or more days before planting out, the plants will have time to replace the pruned roots with new growth at the stem base.
Avoid obvious runts, but at the same time choose smaller, rather than larger plants as they are less likely to be root bound from lack of growing space. If the nurseryman allows it, the experienced gardener will gently lift one of the plants from a cell and inspect the roots. If they are white, the plant is just right to continue growing when set out. Brown or grey roots are an indication that the plant has been in the pot or cell too long for a smooth transition into the garden.
Choose the largest pot available if buying individual plants. Some flowers and fruit set are acceptable, provided the plant is in a pot four to six inches across.
Avoid plants that are either pale yellow or an ultra-dark blue-green. The first indicates that the plant is starved for nutrients, the second that it has been over-fertilized and consequently is a little too tender for the great outdoors.
Gently rustle the leaves of the seedlings to see if anything flies off. This is a particularly useful way to detect the presence of white flies. An experienced gardener will also look under the leaves for any insects that might be hiding there.