The Royal Horticultural Society (RHS) said the plot will highlight how gardens, plants and green spaces can play a role in protecting wildlife, addressing climate change, and providing a more resilient future for people.
The large RHS garden, designed by Balston Agius, led by Marie-Louise Agius, will feature at the world-famous show, which has made a high-profile move to the autumn for the first time ever this year due to the pandemic.
It means the flower show at the Royal Hospital, Chelsea, will be taking place in September in the run-up to the key United Nations Cop26 climate summit being hosted by the UK and set to take place in November in Glasgow.
The RHS Cop26 garden will comprise four themed areas, including a "decline" area, which highlights current damage and negative practices such as a monoculture lawn and paved-over front garden, and an adaptation area, with features such as drought-tolerant desert plants to respond to climate change.
A mitigation area demonstrates gardening that actively supports the environment, including a wildlife pond and a green front garden, and a balance area with a modern environmentally-friendly cottage garden look shows the possibilities of working with nature even as the climate changes.
Ms Agius said climate change cannot be ignored, but everyone can play their part through horticulture - even with just a window box - and she hopes people will take home ideas from the show.
"We need to be proactive about addressing the issues and we hope to show the role horticulture and a sustainable approach to the design of our external spaces can have in doing this.
"This is not an issue for 'others' to deal with - we each have an individual responsibility and opportunity to contribute towards improving climate change," she said.
On the challenge of delivering a Chelsea garden much later in the year than normal, she said the planting used in the garden will be "looking as it should in September" and will be reflective of the summer the UK had experienced.
The RHS says the UK's 30 million gardeners, who look after hundreds of thousands of hectares of gardens, can manage their outdoor spaces to help wildlife, capture carbon, cope with increasingly extreme weather such as flooding and heatwaves, and improve wellbeing.
RHS director-general Sue Biggs said the Government needs to recognise the role horticulture can play in tackling the climate and nature crises.
It should provide funding for research on how horticulture can best help tackle climate change and invest in education and skills to bridge the "green skills gap" and make horticulture a career to be proud of, she urged.
Speaking about the Cop26 garden, she said: "It's got a very strong political message, we want everybody in the country to listen.
"We do particularly want Government to listen because we want to support their environment plan, we are in agreement with them, but they also need to acknowledge the power of horticulture."
She said there is a lot of emphasis on agriculture and forestry, and a feeling that it is too difficult to talk to the UK's 30 million gardeners.
But she said it is not hard to communicate to them that everyone can make a cumulative difference, and she hopes the exhibit will demonstrate to the Environment Secretary and other ministers the power of gardening.
"I do think it's still seen as a poor relation to agriculture. Open spaces, national parks, farming and forestry is big boys' stuff, gardening isn't seen in the same league - and it's time for that to change."
She added that meeting targets on the environment will require bringing together an army of gardeners to help.