Fresh flowers are often chosen as a sympathy gift, but when it comes to bringing them to the hospital, it really matters which flowers you choose. For example, let's see what flowers should be avoided.

① Strongly scented flowers
People in the same room may not like strong scents, so avoid them.
② Flower pot
Because it has roots, 根付く（ねずく）“root” sounds the same as 寝付く（ねつく）“sleep”, and it assosiates with sleeping in the hospital for a long time, so it is better not to bring them.
③Rose
Dark red roses are not suitable as gifts because their color suggests blood and their thorns may hurt people.
④Cyclamen
It is said to be bad luck because cyclamen (shiku) sounds the same as 四九 “shiku”. In Japan, 四 “four” is sometimes associated with death, and 九 “nine” is associated with suffering.

Also, you can just go to a flower shop and tell 「入院した人に送るためのお花を作りたいです。」 (“I would like to send flowers to a person in the hospital.”) and the flower shop staff will choose the appropriate flowers and make them for you.

Flowers are considered gifts to cheer up the sick, but some hospitals do not allow fresh flowers in the hospital. Before you go to visit someone, it's a good idea to check if it's safe to bring them with you.

Cyclamen picture link: https://www.gardenersworld.com/how-to/grow-plants/how-to-grow-indoor-cyclamen/