Rock gardens usually spring from necessity. The rock-bound coast of Maine is a statement of reality. Ledge pokes up through the soil, and unless you want to involve dynamite, your landscaping is going to have to work with it.

“You’re dealing with a little bit of soil and a lot of rock,” Jeff O’Donal told a class earlier this month at O’Donal’s Nursery in Gorham. “You often have about an inch of soil, so you often don’t have much choice in plants.”

That’s why sedum and hens and chicks or sempervivum are so popular in rock gardens. They require almost no soil, will grow in cracks between rocks, and can withstand dry conditions. With both sedums and sempervivum, you can move the plants in the garden by picking them up in your hands with no tools.

They have to withstand dry soil because, except for the few spots of the ledge where the rock forms a natural bowl, the soil that is there is going to dry out quickly.

Another requirement for rock gardens is that the plants be small. Part of the attraction is the rock, and you don’t want the plants to cover all of the rock.

If your rock garden is in the sun, some perennials that will work include silver mound artemesia, the pearl and clip types of campanula, heuchera, Irish moss and creeping thyme.

O’Donal likes Icelandic poppies as a rock garden plant because they are alpine and do well with little soil, and because they come in a wide variety of colors. They are a biennial and self-seed, so they need little care. Another groundcover he likes is Potentilla tridenta, an evergreen that has small white flowers in the spring and foliage that turns purple in the fall.

For shady rock gardens, O’Donal likes mountain lady’s mantle, ghost Japanese fern, Kyusha dwarf meadow rue and a gorgeous tiny hosta called blue mouse ears.

And overall, he would put in some dwarf iris and scilla bulbs, which would show off well in the spring.

But O’Donal is mostly a woodies guy, and the plants he spent most of the time on during his class were shrubs and trees — dwarf shrubs and trees.

When planting a rock garden, he said, you’ll want to put in small plants that won’t crowd each other out in a short period of time. Otherwise, you will have to spend a lot of time pruning the plants or moving them when they get large.

But you have to be careful about what kind of dwarf you are getting. As an example, he used the mugo pine.

“The mugo pine grows in the Swiss Alps, and it is really 15 feet tall and 30 feet wide,” he said. “Anything smaller than that is a dwarf. So you have 6- to 8-foot dwarf mugos.”

A 6- to 8-foot mugo would overwhelm a rock garden, so you are going to have to go with mini plants — plants that grow a half-inch to 3 inches a year. Being a woodies guy, O’Donal had some in pots to show off.

One of the most unusual was “Cole’s Prostrate” canadensis. The version he had was 22 years old. It wasn’t much larger than a beach ball growing in a 3- to 5-gallon pot, and the original was discovered in New Hampshire.

It has a weeping form, so if you were to plant it on the top of some ledge in your rock garden and let the branches droop down in front of the rock, it would be gorgeous. The price, however, is $219. You probably would not want more than one of them.

A couple of pines would also fit well in rock gardens.

Bristlecone pines are great slow-growing plants for a rock garden.

In California, 3,000-year-old bristlecone pines exist that are only 30 feet tall. The needles have white resin dots at the end of them, the only pine that has them.

When O’Donal entered a bristlecone pine in the Portland Flower Show several years back, the judges downgraded his exhibit because they thought it was diseased.

O’Donal’s sells 3- to 4-foot bristlecone pines for $240, and while they have an ultimate height of 15 feet, the people who plant them can let their descendants worry about them getting too big. In 75 years, they might be 6 to 8 feet.

Getting back to mugo pines and plants that an average buyer might want, “White Bud” is a genetic dwarf that would be great for rock gardens. It grows 2 inches a year, will end up 18 inches tall and 3 feet wide, and costs $20.

Blue rug juniper is a common plant, first found on North Haven. But some blue rug at Iseli Nursery in Boring, Ore., was struck by lightning, and part of the plant turned gold.

Cuttings from that plant were taken, and the result is “Mother Lode.” It will grow only 6 inches tall but 5 feet wide, and the price is $26 to $40.

Cotoneaster is not one of my favorite plants, but O’Donal recommended “Tom Thumb,” another true dwarf. It grows only 18 inches tall, will get 3 feet wide, and has dense foliage that turns bright red in the fall.

Most people wouldn’t recommend a hydrangea for a rock garden, but O’Donal said “Little Lime” would work. It will get to be 3 feet by 3 feet and be absolutely covered in blooms by the third year.

O’Donal mentioned a lot more plants than I have room for. But remember: small and low-growing. Weeping is good.

And dwarf does not necessarily mean small.

Staff Writer Tom Atwell can be contacted at 791-6362 or at:

tatwell@pressherald.com