If one of your plants or clematis seems to be declining in health because it has yellow foliage or slow growth or is stunted for what seems to be no particular reason, you might want to consider looking for something that could be lurking below feeding on your plant’s root system. If during your investigation you unearth a stricken plant and discover something that looks like a white, cottony substance (see pictures above) that resembles mold on the roots of your clematis or other plants…this is not good news because you have root mealybugs also known as ground mealybugs (scientific name: Rhizoecus spp.). Unlike many gardening culprits, these are particularly nasty because they are “unseen”, meaning it is impossible to detect them above ground. When they are at their beginning stage of attack they are often mistaken for small pieces of perlite, a medium used found in many soilless potting mixes to help improve drainage.
Just like their more familiar siblings, your regular garden variety mealybugs who see the light of day, they are equipped with a sucking mouth part which they use to feed on the root’s sap. The nymphs of ground mealybugs are also called “crawlers” because they are very mobile and move about until they find a suitable feeding site. Left unchecked, large populations can eventually result in a weakened plant and/or death due to reduced nutrients and water intake.
One telltale sign is to look for ants at the base of an infected plant because they feed on the sweet substance secreted by the root mealybugs. As often happens in nature, symbiotic relationships occur and in this case ants provide reciprocal relationship with root mealybugs. Their part of the liaison, in exchange for being supplied with the honeydew, is to be responsible for distributing them onto other parts of the roots or neighboring plant’s roots in order to encourage more production of the sweet excrement.
Infestations usually begin with new plant material. That is why it is imperative to carefully inspect any newly acquired plant’s rootball, especially if it is root-bound. I couldn’t find any reason why, but apparently root-bound plants are more susceptible to infestations of root mealybugs.
If you discover them on a new acquisition that you are planning to add to your garden, I’d suggest that, unless the garden center you purchased it from has a generous return policy, you might want to think twice and just dump it, especially if it is seriously infested. I know some of you may think that would be a real waste and a rather drastic solution, but these hidden creatures are very hard to eradicate once they become established.
Control your ants! As I mentioned above, ants have a symbiotic relationship with these sugar-secreting allies which means it behooves them to spread these troublemakers throughout your garden, allowing them to continually feed their sweet tooth.
Always add a piece a screening material over the container’s drainage hole(s) to inhibit the crawlers from entering or leaving. This is an extremely important safeguard since the nymphs can easily be transported via water to any uninfected plants in your garden.
Repot your clematis/other plants every 3 years, which is always a good practice anyway. This way you catch them before they get a real stronghold and are able to do irreparable damage.
If you are planning on keeping a lightly infested plant, it’s essential that you isolate it immediately in order to halt the root mealybugs future proliferation in your garden. You must eliminate the chances of their crawlers having any further opportunities to infest your other healthy plants. To achieve this, remove as much of the existing infested potting mix (or soil) as possible and carefully dispose of the all of it in a sealed plastic bag and place it in the trash. Once that is finished, I suggest using one of the following two treatments.
#1 The Hot Water Drench
This technique, as the name implies, uses a “Hot” water drench and is suggested by the Dept. of Entomology, University of Hawaii and it states: “most insects on or in flowers, foliage and roots, including ants, foliar and root aphids, armored scales, soft scales, foliar and root mealybugs and whiteflies are killed at 120° F (49 C) from 5 to 12 minutes (disinfestation treatment)”.
I tried this method out on a clematis (see the picture above) and it did indeed instantly kill the root mealybugs. I can happily report that the thick shoelace roots of my plant were not negatively affected (i.e. scalded). If you are worried that you are going to cook your plant’s roots, the water would have to reach the boiling point (approximately 212° F) and be applied for 3 to 5 minutes for this to occur. Just make sure the water does not to exceed 120° F and you will be fine.
#2 Chemical Pesticide(s)
In your battle, if you are inclined to include insecticides in your arsenal, you should contact your local County Cooperative Extension agent for specific product recommendations of chemical pesticide(s) for dips, drenches, or granulars for root mealybug control. When using any non-ecological friendly chemical(s) always take precautions with any runoff after watering these pesticides in.
Being one who doesn’t run straight for the WMD’s (weapons of mass destruction) when it comes to battling garden enemies, of the two methods suggested, a hot water drench would be the one I would deploy.
Good Horticultural Hygiene
After you have treated the rootball it’s time to replant. Use an unopened package of “sterilized” soilless soil. This insures that it will not be contaminated with any of the crawlers. I never recommend reusing any previously used potting mix, so the possibility of crawlers dwelling in it is another sane reason for adding it to my list of reasons not to use old soil.
If you plan to return your treated plant to its original above-ground home, be sure to sanitize the container using 2 teaspoons of Physan 20 per one gallon of water. Soak for 10 minutes and do not rinse it off. If it’s an old nursery container, I would just pitch it in the same sealed baggie with the old soil.
Some Final Thoughts
Ultimately, after my photo shoot, I decided to dispose of the clematis I gave the hot water drench to. Sadly, despite the fact that the drench appeared to have successfully killed them, I couldn’t stop thinking about even the remotest possibility of more contamination coming from those nefarious nymphs. Be assured, I did a thorough autopsy and all the root mealybugs were more than well done. So, call me a coward or even a clematis assassin if you want, but I decided to be safe rather than sorry.
After doing my research for this article I’ve come to a depressing but realistic conclusion: no matter how much I love a clematis or any other plants, should I find an infestation of root mealybugs I will dispose of it instantly (of course, sealed in an airtight plastic bag) for the good of the rest of my garden. I guess that is what you call “tough love”.