Anna Pavord: In praise of the humble greenhouse

The greenhouse has been a relatively recent discovery for our correspondent. Now Anna wonders how she ever managed without this home for fledgling plants, hippeastrums – and Edith, the pet agave

(Alamy)

The greenhouse is as full now as it ever was in summer. Different stuff of course, but still bursting with plants. How did I ever do without it?

For most of my gardening life, I managed with cold frames. The greenhouse came about a couple of years ago when a gale removed the old tin roof of one of the sheds. The advantage of starting with a shed, rather than a blank space, is the breezeblock walls. On the south and the west side they are 90cm/3ft high, timber clad, with glass above. On the north and east sides, the walls are solid breezeblock.

Greenhouses made entirely of glass cool down very quickly. The blocks work as they do in a storage heater, hanging on to warmth and releasing it slowly. We don’t aim to do more than keep the space frost-free. Last year, after a chilly spell, I bought a heater, a Bio Green Phoenix 2.8kw. It was expensive (£199.99) but came highly recommended. It’s on the lowest setting and I’ve yet to hear it running.

But having power available in the greenhouse was another advantage of re-using the shed rather than starting from scratch.

We dug a narrow border under the solid block wall at the back. This was for a nectarine, which already fills the whole wall. But in the winter, the border is a useful space to overwinter pots of pelargoniums and succulents (the black kind) that are of tree-like proportions and too tall to fit into the cold frame.

There’s also Edith, my husband’s pet agave, who as a baby was winsome enough, striped in cream and greygreen.

She’s now become a problem: spiky, truculent. And BIG. But what can we do? We can’t just leave her outside, howling in the cold, deconstructing in the frost. Having got her into the greenhouse though, will we ever be able to get her out? She’ll fight every inch of the way.

The potted bay trees that I’m growing on from cuttings give no trouble. Until they are at least 90cm/3ft tall, they are safer kept under cover in winter and need very little water. They are ranged along the nectarine border, with two clay pots of lemon verbena (Aloysia triphylla).

We inherited a big plant of this, growing outside against a west-facing wall of the house, but we lost it in one of the recent tough winters. I’ve planted another outside, but the potted plants are a necessary back-up.

Fresh lemon verbena makes the best of all infusions and, under cover, leaves stay on the plant until early December.

When you see growth buds just beginning to start on the bare twigs in spring, you can prune the whole thing quite hard to make a framework for the new season’s growth.

A lead pig-salting trough (185cm x 70cm bought from a scrapyard for £80) runs under the south-facing window, and is full of plants for the house: four pots of freesias, 15 of hippeastrum, four of cymbidium. These are all old plants.

The three new hippeastrum (they used to be called amaryllis) I bought in autumn came into flower before Christmas with outrageous great trumpets of white, red and deep purple.

The ones you keep from year to year flower later – if they flower at all. Some rot off. Some need a year or two to recover equilibrium after being pumped with the equivalent of steroids by the commercial bulb growers.

I used to repot the old hippeastrums in fresh compost each season, but I’ve learnt they do better if they are left undisturbed in their pots. Now I just top up the grit and add a sprinkle of slowrelease fertiliser.

The freesias, having spent the summer dormant in their pots in the cold frame, come into bright green leaf in autumn but are not hardy enough to leave outside. I don’t get more than three or four flower stems from each pot but they are worth growing just for the smell.

I got my first cymbidium about 10 years ago, but they keep multiplying as I divide the old plants and pot them on. Out of flower, they are not worth house room, so they stay in the greenhouse until the flower spikes bore through the clumps of foliage.

Like the freesias, the cymbidiums spend their summers in an open cold frame. They like summering outside, but are not hardy, so as soon as the nights get cold, they need shelter.

Just once, one of the potted cymbidiums produced a spectacular display of seven flowering stems, all at once.

Why? This is the maddening thing about gardening. All those plants had exactly the same treatment, were split from the same parent. And yet this one became a superstar – while the others limped on as usual, squeezing out no more than two flowering stems each.

At least they last a long time. That is greatly in their favour. Proper orchid enthusiasts aren’t interested in cymbidiums.

Too rough. Too unrefined. But some lovely new colours are appearing from recent breeding programmes and if you’ve room to display them, they are undemanding house guests.

Pots of cuttings – yellow Argyranthemum ‘Jamaica Primrose’, two kinds of daisy-flowered osteospermum, scentedleaved geraniums and the spectacular red-flowered Salvia dombeyi – fill a shallow galvanised tray of 90cm x 60cm (3ft x 2ft). As it was such a long, mild autumn, we took the cuttings late. They are set, six or seven of each kind in a 12cm pot, and are now well rooted.

The salvias are for the back of the flower garden where they erupt into dangling panicles of flower in late summer. The rest of the cuttings will fill the pots presently planted with tulips. By late May those will be over and between them, the argyranthemums, osteospermums and scented-leaved geraniums will see us through to autumn.

But somehow I’ve got to find more space in the greenhouse, thanks to an email from Fergus Garrett, head gardener at Great Dixter (and a friend from decades ago). I’d been heaping praise on the agastache ‘Blackadder’ he gave me last year.

“It will strike quite easily from basal cuttings in spring,” writes Fergus. “If you want to be greedy you can lift the whole plant now, cut it back, put it under glass and take two sets of cuttings – the first in early spring after the first flush and another set later (and then take cuttings of the cuttings). I had five plants and made over two hundred. Easy.”