As well as looking for plants with strong healthy growth, plenty of flower buds, and a thriving root system, make sure that they have a full spread of top-growth. Many plants have a \"front\" and \"back\" because nursery staff don't have time to keep turning them to the sun, and the growth may be lopsided. Avoid plants that have wilting or discolored foliage, as well as those with weeds growing out of the soil.
Check the Roots
A good root system is like a well-maintained engine, and if an established plant doesn't have one, don't touch it. First, try to slide the plant out of the pot (not always that easy) and check that there’s a good all-around spread of roots. Avoid pots where congested, restricted root growth means you can't see any soil, a problem known as \"pot-bound,\" and/or when the roots curl out of the drainage holes and make a knotted clump. Being cramped like this invariably signals poor and sickly top growth.
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.
Big Isn't Always Best
It's very tempting to buy the tallest or biggest plant available, but don’t equate size with potential that can be a waste of money. Look for a sturdy young plant with plenty of new shoots and buds as well as a healthy root system that will flourish, instead of one that’s twice the size now but that is clearly going to end up spare, feeble, and gaunt. Give the former a good start, look after it well, and it will soon outperform the latter. This advice applies equally to climbers: the plant with the longest stems may end up producing the most vigorous growth, but equally, it may not. Instead of stem length, look for lots of healthy buds to avoid making an expensive mistake.
Look for Pests and Diseases
When buying from a reputable specialty nursery, it's very unlikely that you will encounter plant problems. At other outlets, unless you check that a plant has a clean bill of health, you risk buying an ailing specimen and spreading pests and diseases around your garden. Hold the plant up to the light and check both sides of the leaves and stems, particularly the new growth, looking for signs of pests. The best way to test for the presence of vine weevils in the soil is to give the stem a gentle pull. If the roots are being eaten, the plant will come away from the soil. Black deposits on the leaves indicate fungi growing on the excreta of sap-sucking insects, so check the plant for aphid infestation. Discolored leaves may be the result of a nutrient deficiency or other ailment or disease.