Yellow leaves and stems on your sprouts indicate they’re not getting enough light. Keep them near the appropriate light source and give them as much as they need (they require longer exposure to fluorescence than they do to natural light sources).

If possible, you should prioritize pulling weeds up by hand. However, be sure not to use your bare hands, as many weeds are sharp or otherwise irritating when they come in contact with skin. Always don thick, protective gardening gloves to protect your hands when you pull up weeds.

Mowing your grass regularly, especially if you’re mowing close to the ground, can help kill Johnsongrass. Frequent mowing helps limit the spread of Johnsongrass and can even deplete carbohydrate reserves in rhizomes over time. The more time you leave in between mowing, the more likely Johnsongrass is to regrow and take over your lawn. 

Very carefully. You should be cautious when using any of the above methods, as any one of them could lead to you accidentally killing desired grasses and plants. However, there are selective herbicides that can target certain plants over others. Look for a product that only affects the type of weeds you want to eliminate and spares the plants you want to keep.