The campaign asks people to post photos of the "fleeting beauty of blossom" on social media using the hashtag #BlossomWatch, according to a press release.
The campaign emulates hanami, the Japanese tradition of celebrating blossom as the first sign of spring, said the National Trust. The charity said it aims to help people connect with everyday nature and lift spirits in uncertain times.
"It's really easy to take this moment in nature for granted," said Andy Beer, nature expert at the National Trust.
"Celebrating blossom is a pivotal, seasonal moment that can often be all too fleeting and we want to do all we can to help people and families at home to enjoy and take stock of a special moment in the calendar."
There are likely to be waves of blossom across the UK in the next three months, added Beer.
Last month, the charity released a report named "Noticing Nature," which showed that fostering closer connections with nature makes people more likely to protect it.
In Japan, hanami centers around cherry trees, which tend to flower in late March and early April, according to the National Trust.
Japan's cherry blossoms are a huge tourist draw and a source of national pride, but at least two of the nation's cherry blossom festivals won't be going ahead.
The Osaka Mint cherry blossom festival has been canceled over concerns about the coronavirus outbreak, according to a statement from the organizers.
In Tokyo, the Nakameguro Cherry Blossom Festival also said it would cancel its event this year, including its main attraction of lighting up trees along the river.