When is the best time to apply pre-emergent?
Q: When do I put down pre-emergent this year? The weather has been all over the place, and I think I’ve seen forsythia blooming for a while now.
A: This can be a complicated answer, because it depends on what weed you are trying to prevent from germinating. Even though a variety of weeds are discovered in spring, only some of those species actually germinate then. Others sprout in autumn as the weather cools and the days shorten, and only ramp-up growth and start to bloom once we hit spring, making them more conspicuous. Since pre-emergent herbicides only suppress germination and don’t impact older plants with established root systems, you’ll need to plan ahead next time for any weeds that sneaked by you.
While forsythia blooms can be a handy memory device to know when crabgrass tends to germinate, as an example, be aware that look-alike shrubs can bloom even earlier and aren’t necessarily indicators of a suitable temperature. Winter jasmine (Jasminum nudiflorum) blooms quite early, sometimes weeks before forsythia, and can easily be confused with that species.
The two most commonly targeted weeds in early spring are stiltgrass and crabgrass. The former germinates roughly two weeks earlier than the latter. An easy-to-use benchmark for crabgrass germination is a three-day period of soil temperatures reaching 55 degrees or above. You can find a link to a soil temperature tracking website by visiting our Crabgrass webpage. Enter your ZIP code to find a chart of recent soil temperatures, plus a fiveyear and 10-year average. For this year, we’re above average (no surprise there), but some parts of Maryland have not yet had the requisite warming for long enough. Even if you miss the ideal window of time, you can still apply pre-emergent to catch seeds that haven’t yet germinated due to colder local conditions or individual seeds that are just late to “wake up.”
Be aware of any local laws regarding lawn or garden pesticides. Notably, Montgomery County and Baltimore City regulate what pesticides (including herbicides) can be used on lawns. Additionally, the Maryland fertilizer law prohibits the application of lawn fertilizer prior to March 1, so products that combine a pre-emergent with a fertilizer cannot be applied too early, regardless of temperature trends. Corn gluten acts like a fertilizer as it breaks down, and applications at a rate that increases pre-emergent efficacy will violate the fertilizer regulations.
mostly brown this winter, when in years past it seemed OK. Did I lose it? We’ve been fairly mild overall this winter, so I’m surprised.
A: Unfortunately, it’s probably toast. We’ve received a handful of queries so far about established, good-sized rosemary shrubs turning brown this season. I had a similar experience — a rosemary that did remarkably well for several years in a pot (great for drainage, but not great for hardiness) recently turned brown.
Rosemary can be fickle like that with our winters — either a cold snap winds up being more than they can handle, or the roots stay too wet due to a drainage issue or a period of excessively wet weather. When we have enough snow on the ground during cold snaps (remember snow?) that serves as a bit of insulation from the worst of the temperature drops.
Some rosemary cultivars are a bit more reliably cold-hardy than others (‘Arp’ and ‘Hill Hardy’ are two), but we’re still at the northern edge of their tolerances overall. Ironically, bringing a potted rosemary indoors for the winter doesn’t necessarily solve this problem. Sure, it’ll stay warm enough, but it will be actually too warm and dry (low humidity) to thrive, plus it’s usually not nearly bright enough. If you are fortunate to have a greenhouse, especially one that’s minimally heated, then that might be the best solution since the ideal winter conditions for rosemary are cool, humid, and sunny. Otherwise, occasionally, we Marylanders will need to replant a new rosemary shrub when an older one is lost to winter.
Don’t want to give up on it just yet? If you see some green, you can always wait to remove the plant to see if there is new growth from still-living branches. The rest will need to be trimmed off as the weather that killed the foliage probably also killed any dormant leaf buds on those stems. For anyone new to planting rosemary, or for the occasion when new plants are needed as replacements, try to site them near a southern-facing wall so they benefit from the few degrees of cold protection that may offer. It may not sound like much, but a few degrees sometimes makes all the difference.