Newly developed cultivars resist, but are not immune to, Dutch elm disease


Our neighborhood lost three large elm trees in the past few years, and we would like to replace them. What can you tell us about the liberty elm? Is it resistant to Dutch elm disease?

My answer was gleaned from several sources, including the liberty elm Web site, If you would like more details, please visit the site.

No American elm cultivar is considered immune to Dutch elm disease, but several cultivars are considered highly resistant. The three most notable are the liberty elm (Ulmus americana 'Liberty') and two cultivars released by the National Arboretum, 'Valley Forge' and 'New Harmony.' These cultivars have emerged from 30 years of plant breeding and disease trials on thousands of plants. All three are excellent growing trees, but some sources indicate that 'Valley Forge' is the most disease resistant.

I support your replacing the elms with the new disease-resistant ones, but it is dangerous to fall in love with any one species. If we plant a variety of trees, we are much less likely to be devastated by future disease and insect epidemics.

After living for many years in Baltimore County, we recently moved into Baltimore City. How much difference is there in winter temperature and frost dates?

It depends upon where you lived in the county. If you are moving from a southern area of the county like Essex or Arbutus, you will have very little difference, but if you are moving from northern Baltimore County, the difference will be significant.

There are several reasons for this. First, the Chesapeake Bay has a moderating influence on winter temperatures. As you move north into the county, this influence wanes. Second, as you move north from Baltimore City, there is an increase in elevation that has a cooling effect. And third, the structural elements of the city (roads, buildings, etc.) absorb and retain heat, which makes the inner city a few degrees warmer.

With these combined influences, there may be up to three weeks difference in the last frost date as you move from the inner city out into the county. Last frost date in the city is around April 20, but it may be as late as May 10-15 in northern Baltimore County. Warmer temperatures are one of the benefits of living in the city.

Dennis Bishop is an urban horticulture educator for the Baltimore office of the Maryland Cooperative Extension Services. If you have a gardening or pest problem, you can call the Home and Garden Information Center hot line (Monday through Friday, 8 a.m. to 1 p.m.) at 800-342-2507. You can also e-mail questions, order publications and diagnose plant problems by visiting the Web site www.


1. If the weather permits, this is a great time to start winter pruning of shrubs. Overgrown shrubs can be reduced by one-third or more to encourage fresh, new growth in spring.

2. Planning a new garden for next spring? Be sure to use a diversity of plants. Diverse gardens are less susceptible to disease and insect problems.

3. Start preparing to plant your summer vegetable crops indoors. Gather pots, soil and other supplies. Assemble a light stand. Seeds can be planted next week.

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