Ketaki, known as umbrella tree or screw pine or keura in Hindi, is barred from the worship of Lord Shiva. It is not offered during pujas and worship as it is cursed by Lord Shiva. There is an interesting story mentioned in the Puranas that explains why Ketaki Flower is forbidden.
Once Lord Brahma and Lord Vishnu got it into a fight over who is supreme. To sort out the issue Lord Shiva made His first appearance in the form of Jyotirlinga or Lingodabhavamurti (a column of fire with no end or beginning) before Lord Vishnu and Lord Brahma. Lord Shiva intervened in the fight and said whoever can find out the origin or end of the Shivling is superior.
Lord Brahma and Vishnu set off to explore the beginning and end of the mighty column of light. Vishnu went down in the form of a boar and Lord Brahma went up in the form of a swan.
Lord Vishnu was unable to find the base and came up and admitted defeat.
Brahma on his journey upwards came across a Ketaki flower. Brahma again went up but was unable to find the uppermost limits. So He decided to take the help of the Ketaki flower.
Brahma decided to take the Ketaki flower back to Vishnu to bear witness that he had reached the top of the pillar of light. Brahma said he found the Ketaki flower atop the Jyotirlinga and ketaki supported it.
This lie infuriated Shiva. Brahma was cursed that for telling lie, He would not be worshipped on earth by people. Similarly, ketaki was also cursed that she would never again be used in worship of Shiva. Thus, ketaki is debarred forever from pujas and worship of Lord Shiva.