Yes, it is true crabgrass seeds germinate at around the same temperature the forsythia flowers need to pop. So when people ask me “when do I but my crabicide down I just say, “before you see the forsythias bloom; and if they are, you’d better hustle yourself down to the crabicide store and get ‘it,’ and a spreader”.
But back to the Forsythia.
Typically, they are a big gangly bush, reaching a shaggy 10 feet by 10 feet, and do best if grown way in the back of the yard. Folks that have tried them in the upfront landscaping usually end up taking them out because of the pruning nightmare they become. Though they have developed a dwarf that grows to 5 feet, and it has worked well in the landscaping, but it may take one more generation to get the bad pruning taste out of our mouths before we move them up closer to the house again.
The Forsythia can also tell us how severe the winter was. If the temperatures were to drop below -20 degrees at any time during the winter you will not see the yellow flowers come spring. But that will not be the case this year.
The Forsythia is another one of those plants that measures the horticulture prowess of an individual; if you kill one of these there is no hope for you, sorry.