Thistles and Fire Blight

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Q: I am having a problem with thistles in my flower beds. I can’t seem to get rid of them. What is the best treatment?

A: The key to many weed problems is timing. Most thistles are biennials. This means that during this spring, summer, and fall, new plants will sprout from seeds. A rosette of leaves will grow a taproot to store energy so it can survive over the winter. Next spring and summer, they will send up a flowering stalk that will send out tens of thousands of seeds. The plant will then die.

The best time to use a weed killer is when the plant is small, during its initial growth phase; that’s when the plant is most susceptible to it. Herbicides won’t do much when the plant is dormant the first winter and are only partially effective the second year as it blooms.

Chopping the rosette off from the root will kill small plants without chemicals. Chopping it off when it is big is less effective, as the root might be big enough to grow new leaves and flowers. Cutting the flower stalk off prevents the seeds, and the plant might die without sending out any more flowers. Sometimes the shorter flower stalk will branch out and send out new flowers, so it may need to be cut off a second time.

Preventing the flowers from producing seeds will reduce the number of plants in the future. As long as you cut off the flower stalk to prevent new seeds, you will win, as the plant is going to die in two years no matter what.

Q: The leaves on the ends of my Bradford pear tree branches are turning black. Is this a serious problem, and can I just prune it out?

A: It sounds like the tree has a bacterial disease called fire blight. The main symptom looks like someone has taken a lighter to the end of the branch, scorching the leaves. Also, usually, the end of the branch will bend down and look like a shepherd’s crook. This disease affects the rose family plants of roses, apples, pears, crabapples, Pyracantha, serviceberry, spirea, raspberry, mountain ash, quince, and cotoneaster, among others. On some of these species, the black leaves and curled branch tips are not as noticeable.

The bacteria survive the winter in cracks and damaged areas of the stems and branches. In the spring, sap and bacteria ooze out of the cracks. The ooze drips and splashes onto young succulent growth, infecting it. It is also transferred by bees and other insects that feed on the ooze before pollinating a flower.

The bacteria move from the flower into the stems, causing the black shepherd’s hook appearance. The bacteria stay in the stem if the plant can wall off that area, or if the plant doesn’t stop them, they move down into the trunk, where they can kill the whole plant. New infections can occur anytime the stem is damaged by pruning, insects, hail, or wind damage, especially if the weather is warm and humid.

Usually, the disease doesn’t spread rapidly, and the diseased branches should be removed in the winter, when new infections won’t start in the freshly cut ends. Cut at least a foot below the visibly damaged tissue. Sterilize the pruning tool between every cut, so you don’t transfer the bacteria to new branches or new susceptible plants. I use a spray bottle with rubbing alcohol.

Pruning in the summer should only be done on plants where the disease appears to be spreading rapidly. Cut out as much of the diseased plant as possible, sterilizing the pruner as you go.

An antibacterial spray labeled for fire blight needs to be applied in the spring when plants are in bloom, long before symptoms appear. Don’t fertilize these plants, as fast-growing succulent tissue seems to be easily infected.

(Courtesy of Jeff Rugg)

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