Q: Some of my creeping thyme has suddenly died out, but the soil doesn’t stay soggy. It lines a path, but we use steppingstones, so I don’t think it got crushed. Will it come back?
A: Hard to say, since several conditions can cause such symptoms (and thyme doesn’t love our heat and heavier soils), but you probably need to replace it. This dieback may be the work of Southern blight, a fungal disease fairly common in our region which thrives in hot weather (above 85 degrees Fahrenheit). It is able to infect many species ranging from annuals (including vegetables and herbs) and perennials (including lawn) to shrubs. As with many ubiquitous pathogens, conditions need to be just right to allow the pathogen to be successful at causing disease, which is why our entire gardens don’t get wiped-out any given year.
While the fungus doesn’t necessarily kill plant roots, the affected plant should still be removed and disposed. (Don’t compost it because the spore structures can be very resilient.) You might even want to scrape out the top few inches of soil near the stem along with the root ball to make sure any surface spores and fungal body (called mycelium) are removed.
When replanting, avoid over-mulching to let plant crowns “breathe” — good air circulation where the stems emerge from the soil discourages infection. Dense ground covers are a great way to reduce weeds and erosion, but this can increase the risk of blight outbreaks by creating stagnant pockets of warmer, humid air. Leaving leaf litter and dead plant stems in place from areas that suffered blight this year is risky, so while plant debris in general can provide wildlife value, it would be best to better sanitize the area around problem spots by removing it at the end of the season. You can learn more about this disease on our various Southern Blight webpages.
Q: I’ve been reading about how to protect soil health and want to use cover crops on my veggie garden this winter since it’ll be vacant. What do you suggest?
A: You can sow cover crops for autumn-through-winter soil protection beginning now, in mid-August. They will germinate and establish, pause growth in winter, and resume growth and bloom in spring (except for oats and forage radish, which are usually winter-killed). At that point you can cut them down to prevent self-seeding and to prep the bed for planting. Leaving the debris in-place will provide a “green mulch” that you can plant through and let decay on its own over the season, nourishing soil life with organic matter without damaging soil structure by tilling.
Cover crop candidates include small grains like oats, winter rye and winter wheat, and legumes like crimson clover and hairy vetch. One crop from each group can be combined in a seed mixture, such as wheat with vetch. Hairy vetch can be aggressive if allowed to go to seed, but you’d be killing it before pods are likely to form and ripen seed.
Legumes provide added nitrogen to the soil as they decay, an extra benefit for vegetables which use nitrogen more heavily than other nutrients.
More information about how to use cover crops and what amount to sow can be found on our Cover Crops webpage.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.