Forsythia is somewhat common shrubs that are a little old school, found mainly in older landscapes. They are deciduous, shedding their leaves during the winter, and sprouting bright yellow flowering blooms with the onset of Spring. Forsythia has always been a good indicator as to when crabgrass germination is near. The lawn care folklore says that when the yellow blooms of forsythia are at their peak, then it’s time to make sure pre-emergent weed-control is applied since crabgrass germination is not far behind.
I have found this to be fairly accurate. Crabgrass seed will germinate when the 4” soil temperatures stay at 54 degrees for four consecutive evenings. The forsythia bloom is more tied to air temperature, but the soil temps rise slowly along with the warmer spring air temperatures. Soils in warmer spots, such as long concrete driveways and south-facing slopes, will warm up sooner in spring, leading to earlier germination of crabgrass. Overall though, in areas like Oklahoma and North Carolina, late March through early April is usually the time when the first flush of germination occurs.
So if your pre-emergent is applied and watered in, as is the case with our existing LawnAmerica customers, you should be in good shape with preventing crabgrass this summer. Pre-emergents are not perfect, but if applied properly and watered in, they do an excellent job of preventing crabgrass and other summer annual weeks from even germinating.
And if you have a Forsythia plant in your garden, enjoy those beautiful yellow flowers for the next week or so. After the blooms are done and your shrub starts to leaf out, it’s a good time to prune your shrub to shape it and prevent it from becoming too large and gangly, as they can if left un-pruned.