Matt Baker and Ellie Harrison are on the border of England and Wales to celebrate the 50th anniversary of Offa's Dyke Path and to explore the fascinating history and treasures that lie along this 1,200-year-old man-made monument. Matt kicks off conservation efforts to help to preserve the dyke and the surrounding landscape for future generations to enjoy, while Ellie abseils to measure and record one of the UK's rarest trees. She also discovers the engineering skills of nature's very own master builders. Adam Henson has got the experts in to calculate the carbon footprint of his farm, and in its 30th year, it's decision time in Countryfile's photographic competition. Which of the thousands of entries will make it into the top 12?

Joe Crowley is on Isle Martin, a dot of an island off the west coast of Scotland, which is getting ready to host Scotland's very first seaweed festival. Joe meets the island's sole occupant, explores the magical qualities of seaweed and sees how it is at the forefront of the battle against climate change. Joe also delves into the Countryfile archives to revisit some island jewels around the UK, including the time Anita Rani visited the Hebridean island where all the farmers are women, Ellie Harrison's trip to Church Island in Northern Ireland, and Margherita Taylor surveying seals off Looe Island. And we catch up with Mari Huws on Bardsey Island, just off north Wales's Llyn Peninsula.