Top of the world: The horticultural high life

Would the supply of green paint last out? That was the big question when we were in Tawang, Arunachal Pradesh recently, just a week before the Dalai Lama's visit.

A decree had gone out from the district commissioner that the whole town should have a facelift, green and white paint only. On Monday, generally the shopkeepers' day off, the whole community was out with their paintbrushes. Along the Old Bazaar Line, the concrete façade of Shree the cloth merchant was turning a brilliant emerald; the faded blue of Rehna, the digital photo studio was disappearing under a coat of the same colour. Restaurants, greengrocers, the Cyber Café, the baker's shop – frantic painting everywhere you looked.

Everyone wanted the place to look its best, of course, but the district commissioner had added an extra inducement: a 2,000-rupee fine for anyone failing to comply. In a country where a tea picker earns just 50 rupees a day, this was a serious motivator. In the centre of the market square, only the superb great bank of prayer wheels remained in its brilliant red, gold and yellow livery.

Tawang, just 39 kilometres from the Tibetan border, was the furthest point north in a journey I had been thinking about for most of the past year. From Sikkim, where we walked for the first two weeks, we planned to move east into Assam (there are direct flights from Bagdogra to Guwahati), and then make our way up to Tawang in Arunachal Pradesh by way of Bomdila and Dirang. You don't have any choice. It's the only road available and even this one route is hanging on by its fingernails to its single-file passage through rock and river valley, working its way up to the Sela Pass, at 14,000ft.

There were many times on that three-day journey north when I thought we were never going to see Tawang. Only the toughest vehicles can cope with the road, but I always preferred to be on foot than in the Sumo jeep we were using. I walked miles along that route, while landslides were cleared, broken suspension repaired, crashed lorries circumvented, stones got out of brake shoes. It's a strange feeling, travelling a road that's actually being rebuilt in front of you.

But you see so much more on foot than you do on wheels. You have encounters that otherwise would never happen, though in this country, it's difficult to explain that you are actually walking for pleasure. Everyone assumes you must be in trouble. Like the army sergeant who stopped us as we were walking through Sessa, one of the many smartly maintained army posts strung along this road north to the Tibetan border: the Assam Rifles, the Sabre Sixth, the Sure Shooters.

We thought we were going to be ticked off, but the sergeant just wanted to give us breakfast at the army Munch Point. "Dhosas?" he said. "Black tea? Water?" And then finally, "Orchids?" That one I couldn't resist, and after introducing us with formality to his commanding officer, he led us down a track to a long series of shade houses, where hand-made terracotta pots with frilly tops were set out in their thousands on wooden benches – cymbidiums, cypripediums, pleiones, dendrobiums, vandas, coelogynes. Perhaps it wasn't so surprising as it seemed at the time: at least 300 species of orchid grow wild in this region and we had seen plenty on our walks, hanging out of teak trees, plastered on to pines. The strangeness came from the conjunction of the army with the orchids.

Moving from sea level up to 14,000ft, we tracked through at least half a dozen types of vegetation, each merging imperceptibly with the next. On the flat, hot plain between Guwahati and Tezpur, palm trees and fields of rice dominated the landscape. As we began our climb into the foothills of the eastern Himalayas, wild bananas and elegant tree ferns burst through the flanks of greenery on either side. On one of the most beautiful stretches, between Sassa and Dirang, the road followed closely the contours of the river, the thick subtropical forest morphing into handsome, buttressed hardwoods, then pine.

Occasionally here, not far from the turquoise turbulence of the river, charging down with its melted snow, a pink-flowered cherry tree, always alone, stood among the predominant green. No leaves, just flowers. Prunus cerasoides I think it must have been. Of the five wild cherries found in the Himalayas, it is the only one that blooms in autumn. But why do we never see it in our gardens? Why does no nursery in the UK supply it? Its flowers make far more of a splash than those of Prunus autumnalis, the only autumn-flowering cherry generally available to us.

The whole road to Tawang was a miracle of hairpin bends, but near the Sela pass, they became even more frantic. The scar made by the road had set off an endless weeping of stone and rock as the mountain tried to regain its equilibrium. Walking this stretch, where the quiet was so strong it deafened you, stones rattled non-stop down the rocky slopes. It didn't seem a great place to be a plant. But it was here, rounding one of the hairpins, that we found blazing blue spreads of a dwarf gentian, which stayed with us for a couple of miles.

What was it? The book I had (Concise Flowers of the Himalayas by Oleg Polunin and Adam Stainton) is geared to the eastern Himalayas, and all the gentians listed there with flowers 2cm long or more grow between 3,000-5,000m. So altitude was not going to be a clue. Flowering time suggested Gentiana ornata.

The Sela Pass is fixed in my mind by these astonishingly brilliant spreads of gentian. And by the wooden hut on one side of the jagged pass, where we got black tea, boiled up from a pan on a small iron stove standing in the centre of the space. Three fat sticks poked out from the front of the stove and a tin pipe took the smoke up through the roof. It was cosy in that tiny place – no windows, and one whole wall stacked with packets of noodles. Who eats them? Who brings them up to the pass? I'll have to go back to find out.