The heavy rains of recent weeks have underlined the importance of good drainage in the garden. Where the soil was well-drained, the water soaked in and flowed away. Where soil was poorly drained, water lay in puddles. That’s not a good sign.

“If you see puddles on the surface, it means you’ve got even more water trapped underground around your plants’ roots,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. “When plants’ roots are in water, they can’t get oxygen, and that limits their ability to function. Eventually they may start to rot.”

Some plants — species that are native to wetlands or floodplains, such as swamp white oak and Siberian iris — can tolerate “wet feet,” or roots surrounded by wet soil. Other species that are native to higher or drier areas, such as pines and sedums, will struggle. Most of the plants we use in our gardens prefer well-drained soil, Yiesla said.

What does “well-drained” mean, exactly?

“Drainage refers to how easily water can flow through the soil,” Yiesla said. “It’s largely a function of how large the soil particles are.”

Sandy soil, which has large mineral particles, is extremely well-drained; water flows right through it. In fact, water escapes so quickly from sandy soil that many plants’ roots don’t have a chance to capture the water they need. Cacti, which evolved where rainfall is scarce, do well…