Garden Docs: French versus Korean lilacs and a bad broom to avoid

Mae asks: What is the difference between a Korean lilac and a French lilac?

Answer: Syringa vulgaris, or common lilac, grows up to 20 feet tall. Varieties of the common lilac are referred to as French lilacs, which come in an abundance of varieties and many colors, from whites and pinks to purples and lavenders.

The older varieties are more fragrant than some of the newer introductions and are prized by lilac collectors.

Korean lilac, Syringa patula, has a denser and twiggy growth pattern. It can be kept to a more manageable height of 3 feet if pruned heavily or will reach a mature height of 8 to 9 feet.

‘Miss Kim’ is a variety that has been in the trade for many years and is known for its pink to lavender blooms.

Justin asks: I travel daily along Fountaingrove Parkway and always admire the drought-tolerant trees and shrubs along the side of the road. Recently I’ve noticed a beautiful shrub in bloom that is covered in bright yellow flowers. Would you happen to know its name? Also, would this shrub be a good choice for my own low-water landscape?

Answer: No! The colorful shrub is a scotch broom, Cytisus scoparius. It originated in Europe and was used for packing material or for making brooms. It has become a tenacious and invasive weed that is almost impossible to eradicate as it thrives among the native plants and overcomes them. If you should see one for sale, or if a friend offers you one for free, don’t succumb to its colorful blooms. Know the botanical name for correct identification.

There are other “brooms” that are not considered weeds nor are they considered invasive. Look under the genus Genista for well-behaved brooms with similar but positive attributes. A favorite of mine is the ground cover Genista lydia. It grows to a height of 1 to 2 feet and a width of 3 to 4 feet, and is covered with yellow blossoms in the spring. G. Lydia requires good drainage such as on slopes, full sun and low water.

Gil writes: On a recent garden tour I saw the most amazing tall shrubs covered in fragrant flowers that were in shades of purple, lavender and white. No one nearby seemed to know this flowering shrub’s name. Is my description close enough for you to identify and give me its name?

Answer: The shrub is commonly, and aptly, named yesterday-today-and-tomorrow. The name describes the color change of the blossoms as they age on the shrub. Its botanical name is Brunfelsia pauciflora. It is a member of the Solanaceae plant family, and is from Brazil. All parts are poisonous and perhaps that is why we rarely see it sold in nurseries.

It seems that this plant was very popular years ago, so you will see it growing in older and established neighborhoods. At maturity it reaches up to 10 feet tall. But it can be kept at a more manageable size. It requires partial shade (such as an east or north side of a structure) and moderate water.

Doug asks: When is the ideal time to prune an olive tree? A gardening friend read that I should prune my young tree into an open center form. My plan is to eventually use the olives for curing, so production is more important than the ornamental form.

Answer: Olive trees are pruned during the bloom period. The “open center form” is the reverse form of a “central leader form.” Think of it as an upside-down pyramid. The open center prevents shading of the lower limbs.

Your young olive tree should initially be pruned for shape and never over-pruned. Over pruning during the first four years reduces tree size, delays fruiting by two years and most importantly, since an olive tree is evergreen, reduces the necessary energy that is stored in its leaves. A vigorous and mature olive tree can be pruned by removing 20 percent of last year’s growth.

Olive trees have a tendency to alternate bearing years, so one can remove more shoots from trees that are loaded with blooms and less to no pruning for those trees with a light bloom.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors, at pdgardendoctor@gmail.com.The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.