Ask a Master Gardener: From a tree disease to crabgrass and shade planting tips

Q. There is black, sticky stuff dripping off of my magnolia tree onto plants underneath, and it attracts bees and wasps. It looks like the tree might have a disease where this stuff is coming from. What is it, and what should I do?

A. This is either the magnolia scale or the very similar tuliptree scale, both of which can afflict magnolias. You are right to be concerned, as heavy infestations can cause twig and limb dieback. Repeated heavy infestations can weaken or kill small trees. Scales are often quite small, but this one can get big, up to inch. These are in the large crawler stage, when their bodies get covered with a white, mealy wax. Fairly soon they will develop a hard shell, turn tan-colored, then give birth to tiny brown crawlers. Those overwinter on 1- or 2-year-old wood, then start to grow again the next year, starting the process all over. Scale are a sucking insect, as are aphids and whiteflies; and all of these exude a sticky "honeydew" that various other insects then feed on. This "honeydew" will also grow a fungus called "sooty mold," which (as the name implies) is dark in color.

Small numbers of scale, at any life stage, can be cleaned off by hand scraping. Their mouth parts break off, rendering them harmless. Wiping off with soapy water can also work when they are smaller. For larger populations, you'll likely need to use insecticides. Killing them in the crawler stage is most effective, as once the shell has hardened, it is difficult for the insecticide to penetrate. If this is a cherished tree, I would suggest you go after the pest aggressively, as follows:

Treat immediately with permethrin (that's the active ingredient), or Spectracide Triazide Insect Killer. The latter is sometimes sold with a hose attachment, which would be convenient if you don't have a sprayer. The former is one of the most common garden insecticides out there, so you just might have some on hand.

Follow up with the dormant oil in November and again in March. This product will have to be sprayed. This is a very environmentally safe and effective control step.

In the spring, April or May, treat with a drench of imidacloprid, which is commonly available as "Advanced Bayer" using the tree and shrub formulation. Because it is a systemic, it is quite effective.

With that, you should see a marked improvement. But you'll still want to monitor the pest and be ready to repeat these steps if it shows up again next July or August.

Q. The crabgrass has gone wild in my lawn. Can I do anything about it?

A. Keep it mowed and wait for killing frost; it is too big and mature to do anything about it now. Crabgrass is an annual, thus is relatively easy to prevent with a number of pre-emergence herbicides (this means the herbicide on the soil stops certain seeds from emerging). After that, when it is quite small, there are just two or three herbicides that can be applied "post-emergence" to give additional control. Examples are Acclaim Extra, Dimension and Tenacity. These would often be used in May, if someone forgot to put down a crabgrass preventer (apply between April 1-15), or mid-June to July, if one noticed crabgrass breaking through. Most home gardeners don't watch their turf close enough to catch this latter situation.

The best way to combat a late-season onslaught of crabgrass (or other annual grassy weeds like foxtail) is to apply a crabgrass preventer a second time, in early June. But if doing this, make sure it doesn't also include fertilizer, as for cool-season lawns; you'll be over-fertilizing, which aggravates diseases like brownpatch. A second preventer application is more likely needed if we have a rainy spring, as this tends to degrade the longevity of the weed suppression. Crabgrass is also stimulated to germinate by sunlight, so mowing at a higher setting will help, as will keeping a thick canopy. Often one will notice crabgrass starting along driveways, sidewalks and bare spots.

Q. I have a shady area that is on a slope. Are there any low-care plants that will give me some color? It would be good if they held the soil, too.

A. Let's work this one backwards. If the area is shaded, then there are already tree roots holding some soil, so one can use mulch or leave a cover of sticks and leaves to lessen erosion. Ground covers that will help hold the soil are generally not too colorful, but you could use a combination of some ground cover and plants to give some color. That said, plants in the shade often don't have as many blooms as what can be grown in the sun, and generally won't bloom over a long period of time. So clustering some of these plants together will increase their impact, and you might try several different ones for more color over the season. There are many different options, and you might try a website to help sort through your options. A good one to consider is the Missouri Botanical Gardens' "plant finder," which is accessible at missouribotanicalgarden.org/PlantFinder/PlantFinderSearch.aspx. If you want more personal interaction, consider a local garden center that has a knowledgeable staff. Contact the Central Missouri Master Gardeners if you need a suggestion or two.

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