Planting under a tree

Q Several years ago, we planted a paper birch in our front garden. Now it seems as if the tree roots are choking out the plants. I don't want to get rid of the birch, but I love my flowers. What can I do?

A Planting a tree in a garden can create several challenges.

As the tree grows, it will produce more shade. That's a good thing if you're trying to grow ferns, woodland wildflowers or shade-loving annuals such as impatiens. It's not such a good thing if you're trying to grow sun-loving perennials. If you want to keep the tree, you may have to put in more shade-tolerant plants as the garden grows shadier.

Another challenge: As the tree's root system grows and spreads throughout the garden, it will increasingly compete with plants for moisture and nutrients. Birch is a shallow-rooted tree, but any tree you plant will develop lots of roots in the upper 10 or 12 inches of soil. These feeder roots are responsible for taking in the water and dissolved minerals needed to maintain good health and growth. The taller the tree, the more widespread its roots.

Here's how to make it work:

• Don't plant too close to the trunk of the tree. Instead, spread a layer of organic mulch (such as wood chips or shredded bark) from the trunk of the tree several feet out.

• Choose plants that thrive in partial shade, rather than sun-lovers.

• To minimize damage to tree roots, start with small plants. Planting perennials rather than annuals also will help.

• When you plant, put a little slow-release fertilizer into each planting hole.

• Mulch around the plants and water the entire area thoroughly and frequently. (Remember, tree roots extend well beyond the ends of the branches.)

Once you achieve the right combination of plants and maintenance, you should be able to enjoy both the tree and the garden surrounding it.

Alternatives to bluegrass Q The Kentucky bluegrass in my back yard always dries up in the heat of summer. I'd like to replace it with something that can get by on less water. I've thought about prairie plants, but most of them grow too tall. I need something that stays shorter, maybe 4 to 6 inches. Any ideas?

A If you want short plants so you can walk on them, you're probably out of luck. Grasses are the only groundcovers that can reliably handle much foot traffic.

Some grasses (zoysia comes to mind) are much more tolerant of heat and dry conditions than Kentucky bluegrass, but they aren't tolerant of cool weather. They won't green up until very late in spring, then they turn brown as soon as temperatures grow nippy in fall. Kentucky bluegrass greens up early, and usually stays green right up to the first snowfall. So, if you're not able to water in summer, the choice is between a lawn that might be brown most summers and one that's brown in spring and fall.

Consider groundcovers

If you don't need to walk around your back yard, if a flagstone or concrete path will do, then there are many groundcovers you could try. Goldmoss sedum (Sedum acre) and wineleaf cinquefoil (Potentilla tridentata) do well in sunny, dry conditions. Barrenstrawberry (Waldsteinia fragarioides) and goutweed (Aegopodium podagraria) do well in shady, dry sites.

For an excellent website on hardy groundcovers (titled Ground Covers for Rough Sites), go to www.extension.umn.edu/distribution/horticulture/DG1114.html.

Keep in mind that most groundcovers will be taller than 6 inches and they tend to be aggressive spreaders -- which is why they make good groundcovers.

Hardy shrubs

Another possibility would be to add shrub beds to the back yard, which will reduce the size of the lawn. Some native shrubs tolerate dry conditions quite well once they're established, as do some old-fashioned favorites such as lilacs and honeysuckles.

Fragrant sumac (Rhus aromatica), bush-honeysuckle (Diervilla lonicera) and sweetfern (Comptonia peregrina) are drought-tolerant shrubs that do well in sun or partial sun. Chokeberry (Aronia melanocarpa) does well in shade.

Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.