Not sure what I am supposed to put in here:

I thought I sent in a question on Saturday, May 25 but have gotten no response. I don't think I set up an account, so am trying again.

We have this little spider web looking stuff on numerous plants in the yard. We put in a plot of ajuga last fall, and alot of it has not come up this spring (maybe 70% did not come back). Several of the plants have the web on it and we wonder if this is harming the plants.

I have found the same webs on other plants in the yard.

what is it, and what should we do about it?

Thanks for your question. We are a bit stumped by the webbing. It's possible that it is caused by spider mites, tiny creatures related to spiders. They are almost too small to see, but you may see tiny dots moving through the webbing if they are present. It is possible that these webs are simply from spiders and unrelated to the plants. Spider mites tend to become more problematic during hot dry periods, and on plants that are already stressed. Regardless, we don't think this is why your ajuga didn't come back.

Assuming that it was irrigated and established well last year, I have a few thoughts. Ajuga is prone to fungal diseases, especially in areas that hold water, are poorly drained, or have poor airflow. This problem is worsened by compacted soils. It is also prone to powdery mildew, which exhibits as a white coating on the leaves. The fact that it died back so severely, possibly indicates that it isn't suited for the site. 

Here is some information that covers ajuga and it's preferred growing conditions.
http://www.gardening.cornell.edu/homegardening/scened732.html

Here is some information on spider mites in case you come across them in the future.
https://ohioline.osu.edu/factsheet/HYG-2012-11

I wish I could tell you conclusively what killed your ajuga over the winter, but it's hard to know at this stage.