Reject plants with poor growth
If an established plant has poor growth, there's a good reason for it. It may come from weak stock, in which case it will never amount to much, or it may have been badly neglected. The fact that it has been well watered on the day you see it doesn't exclude the possibility that it hadn't had a drink for days beforehand, or that it has previously been lying on its side in a shady corner. Fight the \"I’ve-got-to-get-planting-this-second\" bug, and don't buy until you've found a healthier, more vigorous specimen.