I would just break a leaf off and see if it bleeds white sap then its milkweed .
Angie

Thanks Angie. I bleeds white sap, but so does dogbane. That is why identification is difficult.

Thats the only way I can tell sorry I couldnt help more never seen dogbane but your pic sure looks like milkweed if you see yellow bugs on it its for sure a milkweed hehehehe.My poor milkweed has to get sprayed every other day from this pests. Angie

I wish it would get aphids, then I would know for sure. Angie, have you tried removing the aphids with a strong stream of water from a hose? It works pretty well and you don't have to worry about toxic pesticides that could kill monarchs and their caterpillars. Of course, you don't want to wash off the caterpillars when you spray the aphids.

Yep every other day I have to shoot the water to them they really bad on my family jewels milkweed its big so it gets them worst and it hates water I might get blower out wish they made a vacum so you could just suk them bugs up in it like a dust buster.And no caterpillars here yet on my milkweed havent seen even one monark this year sad.

Angie

I have been able to positively identify dogbane, growing near Lake Monona, in Yahara Park, located here in Madison, Wisconsin. It has milky sap, and the leaves look a lot like milkweed leaves. However, the stems will have a reddish tint, that is not hard to see. Your plant has green stems, so I suspect it is milkweed, not dogbane. When it flowers you should be able to make a positive ID. I grow three types of milkweed in our yard, Asclepias tuberosa, A. incarnata, and A. syriaca.

I have looked at your photo, and it's hard to tell. The leaves seem less waxy looking than the larger milkweed leaves. I haven't had much experience with Dogbane, but grow lots of different varieties of milkweed. It's definitely not Sullivan's MW. I am including a link to Monarch Watch's website where you will find photos of various milkweeds and descriptions. You will find leaf photos on this site. Hope this is helpful.

Here is a link that might be useful: Milkweed guide

It looks similar to A. syriaca or A. sullivanti, but it's not either of those species. What do the flowers look like? That would nail the ID quickly. From the growth habit in the July 23rd picture, I'm leaning towards a Dogbane.

I'm thinking dogbane, too, but seeing the flower would clinch it. Also, doesn't milkweed have hollow stems, unlike the solid stems of dogbane?

Follow up: I think everyone that thought the mystery plant was dogbane was probably correct. Monarchs always passed over it and went straight to the plants that I knew for sure were milkweed. Besides that, it began spreading aggressively which is not allowed in my garden. I dug it out as best I could, but it kept popping up everywhere all summer long. I am sure I will have to stay on top of it next summer as well.