Hi, 

What size containers are you using? Perhaps they are too small for what is planted and causing them to dry out too much? You could change the planting medium too. Maybe it needs more organic matter mixed in like compost to retain more moisture in the heat of the summer. 

Using a mulch on top of the container soil is good too and it can be any organic mulch: shredded leaves, wood bark, straw etc. A living mulch would be ok too, maybe a clover (not technically native-but common could work well.) To our knowledge sweet alyssum is not invasive in Maryland (more so out west) so it should be ok to plant in the base of your containers. You just want to make sure not to have something that will compete in the soil moisture. You could plant an edible flower like nasturtiums at the base of the plants or try a leaf green like lettuce, spinach, arugula that would like some shade in the summer under taller plants like tomatoes. If you plant them thick, you can just use them as a clip and eat style for salads instead of allowing them to grow into clumps. Herbs that flower that beneficial insects love would be great as well. 

There are drip irrigation systems you could explore for pots as well. 

We don't think a olla would be sufficient enough to keep the soil moist. Also watering twice a day shouldn't be causing rust unless you getting the leaves wet. Different mushrooms and fungus might grow out of the soil from the decaying organic matter but it shouldn't hurt the plants at all. 

You can explore the following links about container gardening that may help:

https://extension.umd.edu/resource/growing-media-potting-soil-containers/

https://extension.umd.edu/resource/growing-vegetables-containers/

https://extension.umd.edu/resource/planting-vegetables-containers/

Let us know if you have further questions. 

Emily