Answer: The flowers you are growing should be able to last a full week if well watered on the weekend and then mulched. Use about three inches deep layer of organic mulch such as shredded bark mulch. This will help keep the soil cool and more evenly moist, and it also helps add organic matter to the soil over time as it rots down.
Clay soil is good at holding moisture, but once it dries out it can be difficult to rewet it. When you water it, you need to water very slowly and thoroughly so it soaks down deep and encourages deeper rooted plants. Your annual flowers will have roots in about the top six inches of soil so make sure your watering soaks in that deep. After watering, wait a few hours and then dig down and see how far the watger actually went, sometimes it can be surprising.
It is also important to add organic matter to clay based soil before you plant. You could work in some compost, old rotted leaves, well aged stable manure/bedding or milled spagnum peat moss. Use a layer several inches deep and dig it into the top six or seven inches of soil. This will encourage your plants to root to their full depth. The deeper rooting is important because the soil naturally stays moister longer the deeper you dig.
As far as the hanging planters, a week is a long time for them to go without water. The later in the season and the larger the plants in relation to the pot and the hotter/drier the weather, the more often they will need watering even with the water holding polymer added to the soil mix. Using larger pots such as a half whiskey barrel and mulching the surface might help, but a week really is a long time. If there is a caretaker, maybe they could water for you at midweek? Or maybe you can move your hanging pots to a spot in morning only sun with protection from the wind, that might help a little. Another option might be to use some of the self watering planters that have a water reservoir in the bottom of the container.
I hope this helps!
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