I've found these notes which are part of a recent lecture entitled "Making and Running Great Gardens 1700 - 1900", so probably above the level of a typical country gardener but still a useful guide to how a professional Victorian gardener might gain their laurels...

"How did the hundreds of head gardeners in Britain in the eighteenth and nineteenth centuries secure the botanical knowledge, experience and managerial competence required to do their jobs?

A few head gardeners, such as Capability Brown, went on to such eminence as garden designers that their lives are well – perhaps too well – documented. But they were exceptional. A better source for the generality of head gardeners is the series of biographies of them published in the Gardeners’ Chronicle – the main trade journal -in the 1870s. The biographies were illustrated by splendid woodcuts of men such as Anthony Parsons, head gardener at Danesbury Park in Hertfordshire.

I will make more systematic use of these sources in future work, but for the moment they make possible a description of the typical career of a head gardener.

It normally began, probably at the age of 12-14, with work as a gardener’s boy, sometimes followed by a formal apprenticeship. This apprenticeship could be with a head gardener or with one of the nurseries, like Veitch’s, which ran a training programme and employment agency for gardeners. If, on the other hand, the training was within one of the great gardens, the boy would live, with the other younger unmarried gardeners, in a bothy – a rudimentary hostel, usually sited against the north wall of the kitchungarden. Living on site was essential, as the junior gardeners were on call 24 hours a day, ready to deal with urgent tasks such as stoking the greenhouse boilers, protecting tender plants from frost, keeping plants properly watered and securing them against unexpected winds. This was on top of a six-day working week, which could often begin in the summer at 4 a.m.; that was the best time, before the advent of mowing machines, for scything the grass. So this machine was a huge relief for the boys and under-gardeners.

The aspirant gardener, later to rise to the heights of a head gardener, was also expected to become a skilled botanist and horticulturalist. Employers were enjoined to provide a library of books on plants and their cultivation, which the apprentices and junior gardeners were expected to study in their spare time, after working hours. Andrew Turnbull, head gardener at Bothwell Castle, recalled that he had the advantage of working in a garden with numerous well-labelled herbaceous plants and that he would each day, after work, try to memorise 50 of them.

Initial training or formal apprenticeship was followed, for the young gardener, by jobs in a variety of gardens, gradually working up through an hierarchy of posts. It is notable that the biographies show their subjects moving all around the country, from job to job, before achieving their first post as head gardener in their 30s or 40s. Many remained unmarried through their early career, living in what must have been rather unsalubrious conditions – the accounts at Waddesdon provide for payments for fumigating the bothy – before, when they were appointed as head gardener, occupying the head gardener’s cottage which was one of the main perquisites of the job."

The full transcript of this lecture is here
http://www.gresham.ac.uk/lectures-and-events/making-and-running-great-gardens-1700-1900

Earlier this lecture makes the interesting point that the popular TV programme Downton Abbey, "hailed" as a period drama, is totally inaccurate in it's non-portrayal of the gardening staff. Of course the Forest has no such grand house, but there are a few smaller ones which required staff too. The lecture describes one of the first "great" gardens, Wrest Park in Bedfordshire;

"In looking at these topics, we are moving from the world of the garden designer, so often the focus of garden history, to the world of the head gardener. Our image of the gardener may be that of Mr McGregor pursuing Peter Rabbit, but in fact head gardeners were important people, both in fact and fiction. Devotees of Wodehouse will recall the trepidation with which Lord Emsworth approached McAllister with a request that he should cut some flowers. Those of you who watch Downton Abbey will have noted that, among a number of anachronisms, it fails to pay attention to the gardens which would have been such an important feature of country-house life. The cast – far too small, does not contain a head gardener or any gardening staff.

In reality the head gardener would have been at least as important a figure as the housekeeper, second only to the butler. What face should we put to the head gardener?
Wrest Park provides an early example of such an imposing person, important enough to the family to have his portrait painted. John Duell was the head gardener there in the early eighteenth century.
Duell was in charge of a large, though fluctuating, labour force. At the beginning of September 1717 he had 10 men, 3 boys and 3 women working under him, but this had risen by the end of the month to 25 men, 4 women and 4 boys, at a weekly cost for labour and materials of £6-13-3½, plus an allowance for his own food and lodging of 6/-. Duell was in charge of weekly expenditure equivalent to £16,450 today."

This second website discusses C18th gardening apprenticeships from an American viewpoint, it can be slow to load.
http://bjws.blogspot.co.uk/2013/03/black-white-apprentice-gardeners-from.html#rpctoken=...

" The terms of apprenticeship was generally 3 years. But in America, it is apparent that boys as young as 8 were contracted out to gardeners, and they often served longer than 3 years. Custom and laws called for apprenticeship to end at the age when the attained "manhood."
In England, few could expect to attain to the rank either of master-gardener or tradesman, who had not served an apprenticeship to the one or the other. On the American side of the Atlantic, because the trade & gardens were generally less developed, advancement in the field of gardening was more open to those who never served an apprenticeship."

(Continued)

Very helpful, Jefff. Have already met Foley and Phelps, and will investigate some more.
Cheers !