For Romantics, Nature is often a source of respite, both for body and for soul. Further, it is where the intuition ignites and leads man to understanding. A tangible expression of the greater soul of man, Nature nurtures the Romantics.
I think literaturenerd makes a great point about Romantics being inluenced by the world around them. I think it is important to remember that Romanticism arose largely as a reaction to the Age of Reason. During this period, the focus was on categorizing nature and the scientific method. This takes all of the fun out of nature, or so the Romantcis would say. They weren't so much interested in what type of tree or flower they were looking at, but insstead they wanted to capture the awe and beauty of that natural thing in their work, whether it was poetry, a short story, or a painting.
I have to say that I love litteacher's definition of Romanticism ("being in love with life"). I have to add that the Romantics regarded nature as a "being" which influenced how they regarded the world around them. The impact which nature had on the authors of the period is obvious based upon the multiple references to its (nature's) power in the life of man.
Romanticism was a movement that took place before industrialization, but not before urbanization. Cities were growing and things were changing.
Nature, for Romantics, took on special meaning. Nature was a source of inspiration. Nature was a symbol of truth and unadulterated reality. Above all, I think, nature was a landscape without man-made definitions so it had a strong connection to the imagination and to a sense of possibility.
I like to think of it as being in love with life. The Romantic poets see beauty everywhere. They liked to look at the world around them and admire it. They might spend an entire poem describing a creek, for example. They found beauty in the simple things.
Many Romantic poets sought solace and peace in nature, and they also recognized the power of nature and the natural world on man. This is especially seen in the poetry of Samuel Coleridge and William Wordsworth. In Wordsworth's "A Few Lines Composed above Tintern Abbey" he expressly comments on the power of the place and his memories of the place to bring him peace in his otherwise busy life. Coleridge's "This Lime-Tree Bower My Prison" talks about how he is trapped in his forest cottage home but how the memory of another place in nature is providing him with relief from his current misery and illness. There are many more examples like these in their poetry.
Whereas poets in previous eras (such as the middle ages and Renaissance) tended to look to God and heaven as the ultimate sources of meaning in human life, the Romantics were more likely to stress man's ideally close connections with nature, or at least with the more beautiful, lovely, awe-inspiring, and appealing aspects on nature.
We might suggest that many Romantic poets wanted to re-establish a kind of communion between man and Nature. But it would be completely wrong to say that they had the same vision of Nature. As a way of example Wordsworth feels Nature (expression of his subjectivity) whereas Popes has a proclivity to analyze it. Of course they both deal with the same topic but differently. The theme of Nature is very important for the Romantic since it represents a kind of return to origins. We can notice that Nature is often described as a living person. In Popes the English scenery is structured since he aims at creating harmony and order. Despite the aesthetic character of Popes' poems, Nature is always viewed as useful. It is not the case in Wordsworth. Wordsworth yearns for contemplation. I hope that this answer will be useful.
Nature is the key point in the Romantic poetry but the concept of love with it differs from poet to poet. We can examine the concept of Wordsworth about Nature especially in the famous poem ''Lines Composed Few Miles Above Tintern Abbey" where we find three different stages of his love with Nature. The main point is that whatever the idea Romantics have about Nature, it is, indeed, clear that they are strongly addicted to it.