Answer: Unfortunately, based on your description I am not certain why your flowers died. When you plant, set the plant into the soil so it sits at the same level as it grew in the container, no higher and no deeper. Water well to settle any air pockets. Then water as needed to keep the soil evenly moist. Correct watering is the most important thing you can do for new plants.
Your goal in watering is to keep the soil evenly moist like a wrung out sponge, not sopping wet and not dried out. Over or underwatering can kill plants. 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, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
There is no set schedule for watering, it depends on your soil type and on the weather.
Using an organic mulch several inches thick over the root area will help reduce watering needs as well as feed the soil gradually as it breaks down over time. It will also help keep down weeds.
To deter the cat, you could use a spray on or granular repellent labeled for cats. Or, you could try sprinkling thorny branch clippings on the soil surface, or try changing to a different type of mulch. Too, a thorough watering can help discourage the cat because damp mulch and damp soil is not as attractive for digging.
Please check the label on the weed killer to make sure you applied it according to the label directions and that it is safe to use around the particular kind of flowers you planted. Some flowers are very sensitive to herbicides.
I am really sorry about your plants and I hope this helps you trouble shoot. You might want to contact the store where you purchased them if the plants had a warranty.
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