Kate Hudson "couldn't be more in love" with who her son is as a person.

Kate Hudson

Kate Hudson

The 'Deepwater Horizon' star penned a sweet tribute to her eldest son Ryder, who she shares with ex-husband Chris Robinson, as he turned 13 on Saturday (07.01.17).

Captioning an old picture of the pair, she wrote: "Thirteen years ... Today I reflect on how lucky I am to have spent my entire adult years being Ryder's mother. My dear Ryd man! Today you are 13! Hard to believe but I'm staring right at you and my goodness yes you are a teenager! I couldn't be more in love with who you are as a person and who you are growing into as a young man.

"Smart, funny, joyful, tolerant, perceptive, empathetic, loving, you continue to maintain the beautiful attributes of your true nature and I will always be there as your touchstone to help nurture them or rediscover them if they ever get cloudy.

"But watching you independently keep these characteristics close and alive has been a wondrous journey in which I look forward to witnessing every waking hour. Happy Birthday baby!!! 13 is looking pretty awesome so far!!! I love you #RyderIsThirteen #HappyBirthday #ProudMama (sic)"

Meanwhile, the 37-year-old actress - who also has five-year-old son Bingham with her ex-partner Matt Bellamy - previously insisted she is doing the best she can as a mother.

She shared: "I don't try to sugarcoat things. I'll say, 'I'm doing the best I can.' I like cooking, I like presenting.

"I like it to be an experience and [my kids] help me with it, but if they want something after that, I'll be like, 'Look, I'm placing a meal out for you like a king. I can't do everything. I'm going to have to be the OK mom.'"