Regular rains (or watering) is almost always a good thing for our landscape and gardens. However, every time it rains (or we water) we can get weeds. If you ignore those weeds, let them flower and set seed, the landscape can begin to look like a jungle. For every square foot of soil there are many thousands of weed seeds in the top inch, and they are just waiting to germinate with sunshine and water.
In landscape beds, mulches can play an important role in limiting weed seed germination by keeping the seeds in the dark. Once mulch breaks down naturally and sunlight can once again get to the soil, weed seeds are triggered and the weeds are back. Once mulch is re-applied and settles, it should be two to three inches deep. Remember to keep it away from the base of trees and from the crown of shrubs, perennials, and annual flowers.
The better way to manage landscape beds is to apply mulch to a clean bed. Perennial weeds have a good root system (taproot) and will still be able to regrow through the mulch. Spot treatments using an herbicide with systemic properties can be applied to the new emerging foliage as it will translocate into the roots and take care of the weed for good. Repeated hand removal of the perennial weed will eventually remove the reserves and you win!
If the look of an open bed is preferred, frequent very light (shallow) hoeing or soil disturbance is needed to control any emerging weed seedlings before they establish any kind of a root system. If the soil is constantly kept loose after every rain event or when we water, you can exhaust the seed bank in the upper quarter inch of soil and the beds end up being relatively weed free. Wind can always bring in new weed seeds, so no one is off the hook for the season.
Where you have beds of the same kind of plants, like annual flowers, a pre-emergence weed seed control product can be effective. There are still some rules that need to be followed to get good results. Typically, new plantings will need to be left to grow for a couple of weeks to settle the soil around the transplants or if sown from seed, enough time has to pass before the product can be applied. As with the landscape beds, the soil should be weed-free before making any application. If weed seeds have germinated into tiny weed plants, pre-emergence products will not control them. These products work by either preventing seed germination all together or killing the emerging root radical at the point of germination. Another rule that normally applies is that once applied, do not disturb the soil or you break the barrier and weed seeds can once again germinate. When using any kind of product like this be sure to read the and follow label instructions to be sure it will be safe for the flower varieties you have.
The last and least favorite way to manage weeds is to pull them while they are young and have a minimal root system. Pulling a lambsquarters that is only two to three inches tall is easy, but let it get a foot or more in height, and it becomes a two-handed effort and it still may break off and re-sprout.