Ask Mr. Smarty Plants

You are correct, the pictures you find online of Urtica dioica (stinging nettle) and Urtica dioica ssp. gracilis (California nettle) show a tall slender plant with nasty little spines. The difference is, you are mowing those nettles in your lawn, when you mow the lawn. They would rather grow up tall, but you won't let them, so they are doing the best they can to survive. So, what to do? Do you or your children or your pets run around on the lawn barefoot (or -paw)? Is it killing your grass? Do you belong to the school of "just mow it, it's green"? It may be something to tolerate or ignore. A very healthy lawn allowed to grow a little taller than usual will usually shade out and out-compete a weed. It is a perennial, so it will regrow from the roots even if you have kept it mowed low enough to prevent blooming and formation of seeds. 

The Lady Bird Johnson Wildflower Center neither recommends for nor against herbicides, but if you're determined to try to get rid of the nettles, you may have to use some. You need to be very careful what you buy and follow the application instructions closely. The grass in your lawn is a monocot. Most weeds, including the nettles, are dicots, or broadleaf weeds. The "weed and feed" fertilizers that are highly advertised in the spring are designed to kill only dicots.  The problem with broadcasting or spraying something like that is that the ornamental shrubs, flowers and trees in your garden are also dicots. A puff of wind (which we have in BIG puffs all the time in Austin), an indiscriminate aiming of the applicator or just an accidental spill could have disastrous effects on some of your prize plants. Not only that, but watering or rain is carrying excess herbicides and pesticides down to our water supply, where they end up in your glass of water. We would really rather you give the grass a chance to overcome the enemy on its own before you start chemical warfare.

Urtica dioica

Urtica dioica ssp. gracilis