Answer: Watering is done as needed to supplement rain. How often will depend on the soil and the weather and the plants you are growing, so there is no set schedule. Your goal is to keep the soil slightly moist, not sopping wet or saturated.
It is better to water deeply less often than to sprinkle lightly every day. When you water, apply it slowly at the roots so it can soak in. Avoid wetting the foliage. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, water thoroughly and then wait a few hours, then dig into the soi to see how far your watering went. Sometimes it can be surprising. Marigolds usually root down only about six inches. Morning glories are deeper rooted and so even more drought tolerant.
You should also mulch between the plants with several inches of organic mulch. This will help keep down weeds, conserve soil moisture, and also help feed the soil slowly over time as it breaks down. Do not allow it to touch the stems, keep it an inch or two away. Also do not mulch over top of the seeds you planted as it would prevent them from germinating.
It is best to run soil tests and fertilize on the basis of the test results, but you could use a general purpose granular or slow release fertilizer with an analysis of 10-10-10 or similar proportions. Read and follow the label instructions.
Most kinds of marigolds also benefit from deadheading which means taking off the faded flowers. Take the whole flower, not just the petals. You can snap them off or cut them with a sharp pruner.
My one concern is that you mentioned a tree. Both morning glories and marigolds need full sun to grow well and bloom well. Marigolds will tolerate a little shade, but need a bare minimum of six hours of direct sun to really do well. The morning glory needs to climb up something. It grows by twining or wrapping itself, it is not a clinging vine. So you may need to provide a trellis for it. If it is shady under your tree or if they are growing among tree roots, they will not thrive.
I hope this helps you get started, enjoy your flowers!
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