Question
My yard is in Southern California (forgot that you service all the US) so right now the temps are in the 50's during the day. You had mentioned Spring but I wonder since CA is practically spring all year long that if I could start the treatment now?
The geraniums are growing right now.
And what type of grass would you recommend - one that comes in thick as to ward off weeds (as you suggested) but one that won't brown in the winter - or how does that work?
-------------------------
Followup To
Question -
how do i get rid of the geraniums that grow, and are overtaking, my lawn? i have tried pulling them out, but the keep coming back - thankfully they are in my backyard and not in the front. the only thing is that i have dogs and don't want to harm my dogs when killing these weeds/geraniums.
Answer -
Good afternoon Laura:
Unfortunately  you did not tell me your location, so I will assume you are in a cool season grass area.
Wait until next spring until the geraniums are actively growing. There are a number of broadleaf weed herbicides (weed killers) available for use on your lawn. Choices found in garden centers typically include 2,4-D (2,4-dichlorophenoxyacetic acid); mecoprop or MCPP (2-(2-methyl-4-chlorophenoxy) propionic acid); or dicamba (3,6-dichloro-o-anisic acid); with two and three-way combinations available. Check the label and make sure the herbicide is for BROADLEAF weeds.
It may even list Geranium as one of the weeds that it controls. You may have to apply these herbicides two years in a row because the Carolina geranium is a biennial.
Thoroughly read, understand, and follow all information on herbicide labels. Avoid windy days, as these materials can damage many landscape and garden plants if they drift (spray droplets land off the lawn). Also avoid hot days (over 85 degrees F). It's best to have adequate soil moisture, but no rain for 24 hours after application. Don't mow for few days before and after application. Consider spot treating weeds rather than broadcasting weed killer over the entire area. Use caution on newly seeded areas; wait 4 mowings before treating a newly seeded lawn and wait 30 days before seeding an area treated with broadleaf herbicides. Refer to the label regarding any potential hazards and how long you must leave your dogs off of the lawn. Usually there is an 800 number that you can call and they will tell you how long to keep the dogs off the lawn.
Early to mid fall can be a good time to control broadleaf weeds. Control may be good as weeds prepare for winter dormancy and lawns fill-in bare areas created by the weed dying readily in the cooler weather of fall. Spring and early summer applications may not provide as good of control as fall. In addition, warmer temperatures increase the chance of lawn injury. Regardless the time of the year, weeds need to be actively growing for herbicides to work.
A high quality lawn will develop a canopy which will shade the soil surface and discourage the germination of the geraniums.
The minimum height for any lawn is 2 inches (5.08 cm). The preferred mowing height for all cool season grasses is 2 1/2 to 3 inches (6.35 to 7.62 cm). Mowing to less than 2 inches (5.08 cm) can result in decreased drought and heat tolerance and higher incidence of Insects, diseases and weeds. Mow the lawn at the same height all year.
Mow the turf often enough so no more than 1/3 of the grass height is removed at any single mowing. If your mowing height is 2 inches (5.08 cm), mow the grass when it is 3 inches (7.62 cm) tall. You may have to mow a bluegrass or fescue lawn every three to four days during the spring when it is actively growing but only once every seven to 10 days when growth is slowed by heat, drought or cold.
I am attaching two web sites that may be of interest to you. Simply copy and paste them to your web browser and hit "GO".

Carolina Geranium:
http://www.ag.uiuc.edu/~vista/html_pubs/WEEDS/112.html
http://www.ppws.vt.edu/scott/weed_id/gerca.htm

I hope you have luck on your weed control.
Have a good lawn and HAPPY HOLIDAYS!
Floyd McMahon  

Answer
Hi Laura:
Lucky you! Southern California sounds real good right now. I reside in central Illinois and it is going to be 15 degrees by morning. If the geraniums are actively growing then you can go ahead and chemically treat them with a broad leaf herbicide that contain 2,4-D.
Please follow the directions on the package and avoid any drift of the chemical that may harm other desirable broad leaf plants.

You live in an area that is called the warm season grass zone. Choosing the right grass for your lawn ensures that it will be beautiful, healthy and easy to take care of for years to come.
Steps:
1. Select warm-season grasses such as Bermudagrass, St. Augustinegrass or zoysiagrass if you live in an area where summers are hot and winters are mild. Warm-season grasses are less hardy, grow vigorously during the hot months of summer and turn brown in winter.
2. Choose locally adapted pest-resistant varieties, blends of varieties or mixtures of grass types. Your local nursery or Cooperative Extension can help you choose.
3. Select cool-season fescues or warm-season St. Augustinegrass or zoysiagrass for shady spots if you have any.
4. Plant Bermudagrass, buffalograss or tall fescue in dry summer areas.

Zoysiagrass and buffalograss are the hardiest warm-season grasses.
Most warm-season grasses are stiff and wiry, requiring a power-driven reel lawn mower to keep them looking their best.
Overseed warm-season grasses in fall with cool-season grasses such as annual ryegrass to keep the lawn green in winter.

To find your local Cooperative Extension: Copy and paste to your browser:
http://ucanr.org/ce.cfm

A California publication on turf grass selection is as follows:
http://anrcatalog.ucdavis.edu/pdf/8035.pdf

Have a good lawn
Floyd McMahon