Now is a great time to apply pre-emergent herbicide to flower beds as long as you prepare the bed first. You need to make sure all of the perennial and annual broad leaf weeds and grass weeds are removed before applying pre-emergent. Pre-emergent herbicides are a great way to cut down on the amount of many annual and perennial weed seeds in flower beds. Pre-emergent herbicides control a variety of weeds but not all weeds. Because pre-emergent herbicide will not control weeds currently growing in the beds, weeds and grass must be removed before pre-emergent application. Pre-emergent prevents weed seeds from growing and maturing by inhibiting the root system development of the young weed seedling. This kills it before it matures.
There are several brands of pre-emergent on the market including: trifluralin (Preen®), DCPA (Dacthal®), oryzalin (Surflan®), pendimethalin (Halts®) and isoxaben (Gallery®). 
In all cases, careful reading of the herbicide label is important, since not all herbicides can be used among all ornamental plants and certain herbicides require special application techniques. Therefore, clean the beds up first and apply at the proper time.
Using pre-emergent herbicides is not a one and done treatment. It needs to be reapplied about every 3 months, depending on the product. If mulch is going to be used and annuals are planted, a pre-emergent application should be done after mulching. Consider applying one pre-emergent treatment at the end of March or the beginning of April, one in June after mulching, and another at the end of September or beginning of October to control winter annuals. It is best right before a rain. So now it is activated and protecting flower beds from pesky little weed seeds that could be lurking in the soil. It is very important to water in pre-emergent herbicide once applied as this activates it.
Keep in mind that once pre-emergent herbicides are applied and watered in, they create a barrier thus preventing weed growth; if that barrier is disturbed from digging holes to plant or raking, the product will no longer be effective. Also, if annuals are started from direct seeding in the garden, avoid using pre-emergent herbicide in those areas. This would also include any perennials that you want to reseed and spread in your garden.
Source: Carri Jagger- OSU Extension -Morrow COunty