Annual meadowgrass, Poa annua, can be found growing all year round, carrying out its short, six-week cycle of germinating, flowering and setting seed. This being the case, if it isn't appearing in large clumps, getting in the way of other plants or ruining your display, it might not be worth fighting it.
This grass can make itself at home all around the garden, particularly in beds, borders, the lawn and paving. It has fluffy flowerheads that disperse seed and keep the weed coming.
Use a handfork to remove the grass, digging it out from the root. This is best done before the plant flowers and sets seed. Do not compost flowering plants, or it will spread seeds through the compost.
Annual meadowgrass is quite resistant to weedkillers, but if it's appearing on your patio, driveway or between paving (ie. away from other plants), you could try to control it with a patch weedkiller. Avoid spraying weedkiller on a windy day and near other desirable plants.