Healing a Magnolia, Pruning Redbuds and Perfecting Peonies

It's time for more of your gardening questions.

Q: I planted an Alta southern magnolia about a month ago. The leaves are turning yellow and brown, and many have dropped. What should I do? -- Brad Ferrar

AThere are a number of explanations for yellowing leaves. Southern magnolias shed old leaves in the spring and fall. Nursery-grown trees planted in shade will lose foliage to adapt to new conditions. Transplant shock can cause trees to lose many interior leaves. If a tree is planted in an area that has standing water, the problem could be too much moisture. You can save the tree by moving it to a well-drained location. If you purchased it in a container, the roots could have been crowded, not allowing moisture to penetrate the root ball, even in rain. Monitor moisture by feeling roots near the trunk. If they feel dry and it is difficult to get your finger into the soil at the root ball, soak the roots. Water deeply once a week to help roots establish during the first growing season and during dry spells thereafter. Last, sometimes a tree simply doesn't adapt. The company you bought the tree from might offer a warranty. Contact it soon.

What hybrid boxwood with a maximum height of three feet will tolerate shade?

-- Bill Zimmer

In the Washington region, the classic English boxwood (Buxus sempervirens "Suffruticosa") is one I like for its habit and slow growth. Unshorn, it grows with the appearance of clouds and can reach four to five feet over many decades. Little-leaf boxwood (B. microphylla "Green Mound") will mature into a handsome, three-by-three-foot, deep- green mound.

I live in a cottage in Colonial Beach, Va. The foundation is concrete block. Can I pile rocks a foot or so high and wide to camouflage the foundation and begin planting from there? Would this encourage pests? -- Debby Tait

You can use rock mulch as a buffer between the house and plants, about three to four feet wide. Potomac River gravel would be appropriate, considering your location. Use 3/4 - to 1 1/2 -inch rocks, 1 to 2 inches thick to discourage mice and other wildlife from living in it. Place gravel on a professional-quality nonwoven landscape fabric so you can remove stone from the soil if necessary. Look for large stones, and place them on the rock mulch as sculptural elements. Plants to the front of the rock mulch can be used to screen foundation. The most important step when using stone mulch against house foundations is grading soil under the gravel downhill away from the house to keep water from flowing toward the foundation. Keep the gravel weeded to discourage pests.

In 2004, we planted four redbud saplings. Due to sunlight variations from canopy trees, some have grown more than others. One, in nearly full sun, is about 10 feet tall. The branches on its south side are so full that they are heavily bent. The others have leafed out and have strong, thick branches. Should we prune all the trees or only the weak ones? -- Michael P. Panopoulos

Eastern redbuds are well known for their shade tolerance but also thrive in full sun. They also flower more when in sun. Prune selectively, matching the heavily leafing redbud to the others, allowing more sunlight to reach other trees. Remove the greatest number of branches from the fullest tree, thinking about future growth: When trees are 15 feet tall, will they create a flowering grove?

Prune the weakest branches if it doesn't ruin the tree's shape. Remove deadwood and crossing branches that rub together, and prune branches growing toward the inside of the tree. Cut back to the juncture with another stem. Leave the wider root collar (bottom quarter-inch of branch) to quickly heal. Begin to limb up plants only as they get tall enough to be considered trees (10 to 12 feet). Elevate the bottom couple of ranks of limbs. Ultimately, you want to be able to walk under these trees, while allowing a natural shape. Allow the tree in the sun to remain the tallest. If the others stay too shaded, root-prune them and transplant them to a sunnier location in fall 2009. Space redbuds about 15 feet apart.

What can I put on my moss garden to eliminate grass that keeps popping up? I enjoy pulling the other weeds by hand, but the grass is frustrating. -- Bill Weinhold