There are some colours that burst on us once a year, without precedent: unique, exciting and transient. The new leaves of lime trees have the stage this week; a colour that by coincidence we tend to call lime-green – referring, though, to the citrus fruit rather than one of our champion park trees. Each April I amble round Kensington Gardens, pausing at tree after tree to enjoy the glowing tenderness of the unfolding buds, the little shining package of the leaf pushing out, limp for a day before breathing gives it strength to open to a pale green oval.

Limes were the chosen show-trees of the Georgians. They imported them in thousands from Dutch nurseries to plant their avenues. Kensington Palace has a great crinoline of them centred on the Round Pond, planted perhaps by Queen Charlotte, replanted in the past twenty years or so and now brilliantly verdant, the epitome of a stately spring.

Our little box hedges seem to be almost the last of this threatened species in the neighbourhood. The box moth caterpillar or the blight has either put paid to the rest or their proprietors have despaired of their survival, rooted them out and planted one of the proposed substitutes. Rassells Nursery just across the road follows RHS advice and proposes Ilex crenata, which superficially resembles box. Some nurseries have even cooked up a cunning new name and sell the little holly as Luxus.

It has an upright growth, unlike box, but can be clipped to grow into a flat-topped little hedge that will grow (they say) no faster than box. It may not be as tolerant as box to either drought or too much moisture but is apparently subject to a nasty-sounding black root rot. Nor does it have the slightly foxy smell in the rain that some people hate (and I find part of box’s charm).

For myself, I will go on defending our box hedges, the very definition of our garden, while I can. We use moth-traps and Py spray. We pick off the caterpillars and squish them as soon as we see them. I fear it can’t go on for ever, but while we’re around our box will have all the protection we can give it.

Sixteen degrees on February 16 doesn’t sound or feel right. The magnolia in the front garden looks ready to burst its buds. Primroses are even fading on the north side of the house. Daphnes are in full fragrant flower and the flowering currant is already giving off its Marmite smell. Camellias are in full bloom and in the water tank the fish look distinctly frisky.

Last night we slept with the French windows open. We see summer blouses on the street. The weather forecast shows no hint of winter returning to duty. Should we be alarmed? And is there anything we can do?

The horticultural answer is yes: take advantage. And take precautions. Bring the swelling buds into the house to enjoy them, and try to stop the greenhouse from becoming a hothouse by shading during the day and ventilating day and night.

Now its February 20; the mercury has dropped three degrees and low clouds have hidden the sun. The magnolia has shed all its furry bracts, leaving its purple flowers closed but naked like dark flames among the grey branches. The cercidphyllum that hangs weeping over the pavement is opening its first tiny heart-shaped leaves at the tip of each branch. Crocuses that have found their way into the front garden beds are starting to clump up and a fresh crop of purple flowers are pushing up through the tangle of leaves of Iris unguicularis. It’s a badly designed plant; its leaves succeed in hiding its flowers.

A fat bag of mulch has just arrived form Rassell’s nursery across Earls Court Road. Spreading it is my next job.

Trees may be slow but forests are fast. That’s how it seems to me in our Welsh woods. We were honoured last year with a visit from the Royal Forestry Society (or at least its North Wales Division), and I recalled how things have changed in the 29 years we have been in charge.

What is still recognizable are the contours; slopes of five hundred feet or so from the valley to a ridge, commanding a huge view across the sea towards the Wicklow Hills, and south to the rocky north face of Cader Idris (a mountain more noted for its bulk than its height). That is the airy mountain component. The rushy glen is the course of the Afon Dwynant, from a few springs high on the hill to a burbling, occasionally rushing stream as wide as a country road that eventually tips into the estuary of the Mawdach. The estuary is lined with what is termed ‘Atlantic Oak Woodland’, a precious zone where contorted oaks thrust up from a bed of boulders with only moss and ferns for company. Last year there was no rain for week after week. The hillside springs were dry, with a surprising effect: without moisture they appeared as brown, even scorched, patches in the grass and heather.

The rotation of a commercial forest (as much of ours is) is about sixty years. The tallest remaining trees from the last major plantation , in the 1990s, are spruces almost a hundred feet high and larches, perhaps eighty feet but incomparably beautiful, their delicate canopies soaring on ramrod trunks to form airy colonnades.

Foresters talk about ‘yield-class’, a figure denoting the number of cubic metres of timber produced in a year per hectare – depending, of course, on many factors. Inevitably, sadly, the star performers are always the aggressive Sitka spruce, black in the landscape, spiky to touch. They don’t seem to care if it’s bog or rock; you don’t even have to plant them; their self-sown seedlings sprout everywhere. Foresters say ‘re-gen’, often so dense that it needs ‘re-spacing’.

Our policy is to leaven our money-making blocks of Sitka with admixtures of other conifers, usually larch, or a scattering of broadleaves, which could be beech, birch (which comes up anyway), alder, wild cherry or even Norway maple. Oak is too slow in getting started. Larch, tragically, is suffering and dying from a virulent form of Phytophthora. The lovely Western hemlock, with drooping sprays of bright apple green, is prolific, and can be vigorous enough to hold its own – though sadly sawmills, the market for all our trees, have problems using it and prefer the conventional spruce. Then there is birch and rowan, the sweet-smelling gorse, and of course bracken and brambles. The forest is never dull.

Sixteen degrees on February 16 doesn’t sound or feel right. The magnolia in the front garden looks ready to burst its buds. Primroses are even fading on the north side of the house. Daphnes are in full fragrant flower and the flowering currant is already giving off its Marmite smell. Camellias are in full bloom and in the water tank the fish look distinctly frisky.

Last night we slept with the French windows open. We see summer blouses on the street. The weather forecast shows no hint of winter returning to duty. Should we be alarmed? And is there anything we can do?

The horticultural answer is yes: take advantage. And take precautions. Bring the swelling buds into the house and try to stop the greenhouse from becoming a hot house by shading during the day and ventilating day and night.

Now its February 20; the mercury has dropped three degrees and low clouds have hidden the sun. The magnolia has shed all its furry bracts, leaving its purple flowers closed but naked like dark flames among the grey branches. The cercidiphyllum that hangs weeping over our boundary and the pavement is opening its first tiny heart-shaped leaves at the tip of each branch. Crocuses that have found their way into the front garden beds are starting to clump up, and a fresh crop of purple flowers are pushing up through the tangle of leaves of Iris unguicularis. It’s a badly designed plant; its leaves succeed in hiding its flowers.

A fat bag of mulch has just arrived form Rassell’s nursery across Earls Court Road. Spreading it is my next job.

Mist has gathered in the valley where sheep dot the emerald green with white. High on the hill it blows in scraps and longer straggling clouds across the black of spruce and fir. Between the two sun is reaching a tapestry of trees, finding bright gold in the larches, richer, deeper gold in maples, bone-grey in ashes and a lively yellow-brown in the swathes of bracken. Bracken in the sun is the dominant eye-catcher, and seems to grow more dominant every year.

You get the feeling, sometimes, that bracken will one day smother everything. Yet it’s scarcely a new invention. The diary of the farmer at Sylfaen next door in the 1870s speaks of the bracken-crop in autumn as one of the worst tasks of the year, It was used for animal bedding, but gathering it meant breathing in clouds of its spores, which turned out to be carcinogenic.

Bracken is apparently a universal weed. Antarctica is the only continent without it. It can grow up to ten feet high and spread with its rhizomes hundreds of yards. It won’t grow in boggy ground, so the drainage of uplands only encourages it, but is otherwise unfussy about soils. It laughs off authorized weedkillers. Its roots survive the sharp trotters of sheep. Clodhopping cows are better at breaking them up, but only repeated trampling year after year suppresses them. So expect to see more of it somewhere near you.

Not many people are lucky enough to have time and space to plant an arboretum, let alone nurture it until the trees reach maturity. The fact that I am now on my second argues the devil’s luck – which I happily acknowledge. My first (to deserve the name) is growing, flourishing indeed, at Saling Hall in Essex, where we lived for 42 years. The second is taking shape at our daughter’s house on the fringe of the New Forest and only two miles from the sea. (That is not counting a much earlier effort on the North Downs in Kent; nothing to see here, as Constable Plod would say, but evidence of an urge to put roots in the ground; roots that would lead in time to a contribution to the landscape.)

Sceptics have often told me that nothing much would happen for so long that I shouldn’t bother. You can argue that a tree doesn’t merit the name until you can stand in its shade. Nonsense, I answer: I want to see its leading shoot up close, at eye level; see which buds open in what order, watch the shoot with the most vim take the lead, defy gravity and head straight north and the others accept their fate and become mere branches.

It was John Claudius Loudon, by the way, who coined the word arboretum. Who indeed originated many of the terms and ideas we now accept as fundamental to the language of gardening. Every time I pass his house in the Bayswater Road (number 3, Porchester Terrace) I remember this extraordinary work-driven Scotsman, traveller, designer (of greenhouses and cemeteries) and all-round gardening visionary. His encyclopaedias, two bricks thick, and The Garden Magazine, which he founded and ‘conducted’ from 1826 to 1844, laid down the ground-rules of Victorian gardening – many of which survive in essence to our days. His wife Jane’s contributions were just as impressive. Loudon lost his right arm and was a rheumatic invalid all his life, the son of a farmer in the Borders who achieved more for horticulture than anyone of his time.

There are two plants I’ve kept in pots for longer than I can remember. Even the same pots. Both of them start to strut their stuff at this time of year, when the garden is winding down. Nandina domestica doesn’t have a traditional garden name,  becomes a flourish of narrow red leaves, and Meyer’s Lemon’s lemons ripen to bright pale yellow among its glossy green leaves. The lemon has occupied its handsome Tuscan pot, decorated with moulded lemons, for more than 35 years, its soil unchanged, except for the two inches of compost I dress it with in winter. It hasn’t seen the sun for the past ten years, being on our north-facing verandah. I water it weekly in summer, scarcely ever in winter. The fruit I picked last week, thin-skinned but juicy, was as close as lemons get to sweet.

The nandina, Nanten in Japanese and Sacred Bamboo in America, was a berry in the botanical garden at Kobe, on the south coast of Japan. I picked the berries because they were yellow rather than the usual red. The nanten is Japan’s doorstep plant, the traditional welcome, standing on the stone threshold that is kept wet, furnished with a fresh pile of salt, to welcome visitors. Superstition says the salt wards off evil spirits. Another story is that it attracts the draught oxen that once pulled important people’s wagons. The salt, they say, signalled complaisant ladies within.