TheGardenLady received this question about marigolds.
My marigolds are planted around the edge of the garden and watered 2x a week with the rest of the garden. After a month they just started dying with no visible infestation. Do you have any idea why? Last year I planted marigolds in the same vicinity under a citrus tree and they survived just fine. The plants were bought at Lowe’s garden center both times.
Marigolds are one of the easiest plants to raise. But like any living thing, they can have problems. See here. You write that you bought plants at the same local store that you bought them last year. Last year they might have been healthy but this year you might have bought home unhealthy plants- for example, the pots may not have been watered property and you might not have noticed that they looked sickly.
We are lucky to have inexpensive garden centers to buy plants, but many times these stores cannot find knowledgeable people to work in the nursery, people who know how to properly maintain the plants they sell. In the tiny pots the plants come in without proper watering, the roots might have dried out. Or the plants might have had a fungal or bacterial problem in the soil.
You said that you planted the marigolds in the same vicinity as you planted this year. There can be differences in areas that are close by. For example, in my area in my soil we have lots of shale. A part of the ground close by can have larger rocks down below that doesn’t allow for good drainage in one spot but just a few feet away it is fine.
You said that you watered the plants 2 times a week. Not seeing your marigolds, this seems to TheGardenLady to be what caused the death of your marigolds this year. Marigolds may like a little water when the plants are young-if you raised them from seed. But you bought the plants. After the first watering when you planted them, they really don’t want or need to have you water them unless there is a 10 day or 2 week drought. Perhaps the plants in the border that you put the marigolds in need watering twice a week in the area you live, but this was probably too much water for marigolds. Marigolds like to be in dry soil. They don’t like to sit in wet soil and they don’t like overhead watering. When you water them, they prefer it if you use a soaker hose.  See here.
Dig up the dead plants and discard. Replace them with plants that need as much water as the other plants in your border. How will you know which plants are best? Go to a smaller, local nursery where the people who work there are plant lovers themselves. There are many of these small nurseries in every area. Ask questions when you buy the plants. Tell them what other plants are planted near by to see if the new plants you are buying are compatable.
TheGardenLady has written other posts about problems with marigolds and caring for marigolds. Check them out here and here and here and here.  Hope you solve your marigold mystery.