Denise Richards Honors Daughter Sami on 18th Birthday After Noting Their 'Strained' Relationship

Denise Richards is making sure to celebrate her daughter Sami on her birthday.

On Wednesday, the Real Housewives of Beverly Hills alum, 51, paid tribute to Sami in honor of her 18th birthday with a sweet message on Instagram. In the post, Richards shared a recent shot of her teenage daughter along with a few pictures from her childhood.

"In a blink of an eye you're 18…. !!!! Happy Birthday my beautiful Sami. I love you so much," writes Richards. "Privately many more words to share with you. I love you unconditionally and I am very blessed & grateful to be your mom. And Nana is looking down celebrating with you🎂I love you my sweet Sami💗 Happy 18th Birthday!🎉🎂🎊🎁😘"

In the comments of the photo, Sami, who continues to live with her dad Charlie Sheen, replied, "Thank you so much mom i love you too ❤️❤️." She also shared Richards' post to her Instagram Story.

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RELATED: Denise Richards Admits Relationship with Daughter Sami Is 'Strained': 'It's Very Difficult'

Richards' birthday post for her daughter comes one month after she shared on SiriusXM's Jeff Lewis Live that the pair have a "strained" relationship.

"Honestly, I have a strained relationship with her," admitted Richards, who also shares daughter Lola, 16, with ex Sheen. "It's very difficult. I know we'll get back to where we were eventually but right now, it is strained."

Denise Richards/Instagram; Inset: Steve Granitz/WireImage

Asked whether Richards was happy about Sami's current living situation, the actress said, "Obviously I would love for her to live with me. She lived with me all these years."

"But I think it's very difficult raising teenagers now and especially in Los Angeles when there's access to everything. We didn't grow up with Postmates and Uber where you have everything that you want," she continued. "There are certain rules and I enforce them. And [at Sheen's], there's different rules at that house and that's okay."

Rich Fury/Getty; David M. Benett/Dave Benett/Getty

Back in October, a source told PEOPLE that at Sheen's household, Sami and Lola tend to have more freedom, whereas Richards and husband Aaron Phypers set more rules at their home. (Sami, in September, referred to Mom's place as a "hell house" where she felt "trapped" before moving in fully with Sheen.)

"She would love for the kids, when she has to film, to stay in her nice house but they go to 'Wonderland' where they can get anything they want," said the source, who claims that "everything is a big party" at Sheen's house.

A separate source, however, said, "Charlie is a terrific dad and his daughters adore him. They enjoy being with him. Charlie is a strict father but fair. The girls have rules at Charlie's house, which includes homework and curfew. The girls are polite and smart and funny and a joy to be with."