The early tulips are a good indicator, or the budding of the daffodils (blooming is just a titch late, but even if you wait until then, pre-emergent works). If you happen to (a long shot, I know) have "King Alfred" daffodils, they bloom at almost exactly the same time as the forsythia.

Most of the mid to later season tulips are too late, so...let's try to get some general guides here.

Crocus--way too early, but if you absolutely have to with no other guide, time your pre-emergent about 3 weeks later.

Early tulips--perfect timing

Daffodil--a little late

Hyacinth, muscari, most mid-spring plants--a bit late, but perfectly fine

Later tulips--late, but you can certainly still apply

English hyacinth--very late, but again, sure, apply it if weeds are still sprouting

Using nature as a guide isn't always perfect (plants will respond to air temperature once sprouted more than ground temperature), and of course, spring weather can be whacky. But it's a good enough place to start if you can't monitor soil temperatures. Pre-em should be applied as soil temps start to approach fifty.

https://www.greencastonline.com/tools/soil-temperature

morphedspa. Thank you so much. Your reply is a very good guide.

1. question from what you mentioned: SOIL temperature.

Do you test in the morning, noon, or evening, or all of the above then average out??? The link you provided shows the soil temperature in my area is in the high 50 already, :(. I knew I missed the perfect timing, for all the spring weeds already came up

2. speaking of spring weeds, I have never had good luck applying pre-emergent in the fall to prevent spring weeds. I searched and searched online resource to have some indications (like forsythia in spring for fall weed prevention), yet everybody seems to talk about spread pre-emergent in the spring to prevent summer weeds but never about preventing spring weeds by applying pre-emergent in the fall.

What indications again from the nature or SOIL temperature or else to apply pre-emergent during the fall to prevent spring weeds.

Thank you

1) If all the spring weeds already sprouted, and Greencast says it's already in the high fifites, it's late. :-) But you can still apply, just realize you're already past the first sprout. Normally, you'd test the surface temperature around noon, or just use the Greencast tool's data. From the sound of things, your app date would be mid-February, but could be even earlier.

2) It varies. A lot. I app in fall, but live in a place with a distinct ground freeze every year. If you don't, the clock never really stops on the decay of the pre-emergent and whether it lasts until spring is questionable. Shorter-lasting pre-emergents still wouldn't do well, even where I live in Pennsylvania, and I use Prodiamine at a fairly high dose (7 months of coverage) in September to last through the following August or so (the clock stops in December-March).

If the ground doesn't freeze completely, or it's inconsistent (as it sounds like it might be if mid-February is already time for application), then I'd apply in spring instead for better, more-consistent coverage. If you're also trying to confound fall-sprouting weeds like P. annua and P. trivialis, then you can also reapply in August or early September to cover those.

I'm thinking we've been over thinking the spring application of preemergent. It lasts for 3 months. Put it down long before you need it, and then again in 3 months. For my area I'm thinking January 1st is a better time than waiting. I had weeds a month ago at the end of January.