Sorry to hear about your plant problems. When a shrub dies so suddenly it is almost always due to a root problem. The most common problem is soggy soil conditions or poor drainage. When soil stay very wet the roots may be deprived of oxygen and root rots have a easier time entering the roots to cause dieback and possibly death of the plant. Dry soil conditions on a new plant can also be deadly but your description seems to eliminate this. One other possibility is that damage from a weed and feed fertilizer used nearby may have affected the plants. This is most common when such a product is overapplied and then a rainy spell washes it to the plants or into the root zone. You did not mention the amount of sunlight, but sweet broom likes a lot of sun and preferably primary morning sun although afternoon is fine too. I suggest you dig down a few inches around the plants to feel the soil to determine how much and how often to water. If you can maintain even moisture during this warm summer period the plants will have their best chance of recovering and lossses can be kept at a minimum of what has already died.
I hope this helps. Thanks for the question. Please stop in again anytime we can help with plant related questions.
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