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Going potty: Anna Pavord's greenhouse is getting a bit crowded

Our greenhouse, now just over a year old, evolved from a shed that was already in place, so its dimensions (roughly 4m wide by 2.65m long) were pre-determined. To a great extent, so was its function. I wanted to grow a nectarine on the solid back wall, which faces west. And I've got more and more interested in growing plants in pots to bring into the house. But until the arrival of the greenhouse, I was hampered by having few places to keep them while they were revving up for their performance. Fortunately, the south facing side of the new space has proved to be an ideal place in which to bring them on.

But I also wanted to grow tomatoes and cucumbers in the greenhouse and use it to extend the season for salads and tender herbs such as basil and coriander. Flowers that I grow from seed – ammi, orlaya, cleome, cornflower, marigold – have to stay under cover until the nights are less cold. So over the past week, there's been a great turnover, to make space for the summer set of plants. I'm still desperately short of room.

Rhododendron 'Fragrantissimum' and its cousin 'Lady Alice Fitzwilliam' overwintered in the greenhouse, bloomed on the hall windowsill last month and are now standing in a sheltered space outside where they can stay until late autumn. The three tubs of cymbidiums are still with us in the house. Some of the hippeastrums I saved from last year don't look as though they are going to flower this season, so they've been banished from the greenhouse to the cold frame. The cornflowers I sowed last autumn have just been planted out in the flower garden. So, more riskily, has an autumn-sown batch of ammi. It's not as tough as the cornflower, but they were outgrowing their second lot of pots and were already more than 30cm (12in) tall. So far, the risk has paid off. I've gained room in the greenhouse and they haven't been frosted outside.

Autumn sowing of certain annuals such as these gives you a head start in spring. The plants flower early and then they can be replaced with another batch raised from spring-sown seed. I used the cornflower 'Blue Diadem' (Thompson & Morgan £2.19) which grows to about 75cm (30in) and is wonderfully prolific.

The greediest plants, in terms of space under cover, are the argyranthemums, grown from cuttings I took last autumn. All of them quickly outgrew their initial 5cm (3in) pots and are now in much bigger, deeper containers. I've been pinching out the top shoots, but this doesn't stop the plants growing. They just burst out with new shoots lower down the stem. Of course I should be grateful. And I am. But somehow I've ended up with 14 pots of them.

Some of the argyranthemums will go into the tubs in which tulips are presently growing. As they are tender, I can't shift them outside until next month. There are another 15 pots of osteospermum, also grown from cuttings, as well as 20 pots of the variegated pelargonium 'Lady Plymouth'. Over-production is obviously stitched into my genes. It's the same impulse that makes me fill the freezer with spare red onion and rosemary tarts and make enough batches of marmalade to fill an entire shelf at our local supermarket.

But the annual Lifeboat plant sale is coming up in the middle of May. By then, I'll have a clearer idea of what I actually need here, and the rest can go. They'll need to. The shallow lead pig-salting trough (it's 185cm long by 70cm wide) in which all these plants are presently sitting has to be emptied out ready for the big pots of tomatoes and cucumbers that grew there very happily last summer. The trough came from a local junk yard and although it cost £80 it has been invaluable. There's a hole with a bung in the back corner, so I can flood the trough when I need to water and feed the plants – I put a splash of Tomorite in the watering can – and then drain away the spare water. The floor of the greenhouse slopes, so the water eventually runs out through a hole in the bottom of the wall.

Every flat space is filled with plants. On the wide, west facing window ledge there are five pots of Salvia dombeyi, grown from cuttings I took last autumn. It's a phenomenal sight in bloom, sometimes 10 feet high, with blood-red flowers hanging in bunches. But it is tender and it flowers very late – even for a salvia. The plants already need to be big, when they are set out, otherwise they don't get to flowering size before they are felled by the first frost. These cuttings have already been potted on twice, but they'll need even bigger pots (and more space) before it's safe to set them out.

Squeezed alongside them are nine pots of rosemary cuttings. I'm filling the front of the bank with different rosemaries, most of them snitched from cuttings. Unfortunately, some of the younger bushes died as a result of this last winter's freezing temperatures. I need replacements.

On the tall shelf that runs the length of the greenhouse, just under the glass roof, I have pots of seed waiting to germinate. Cucumber seedlings, sown in mid-March, are already pricked out into individual pots – 10 of them. Half are long cucumbers ('Tiffany', Thompson & Morgan £3.99) half short ('Zeina', Thompson & Morgan £5.99). I grew 'Zeina' last year and it was superb: both productive and full of flavour. There is still plenty of time to sow.

Underneath there's a marble worktop roughly 150cm (5ft) by 60cm(2ft). Marble sounds too grand for a greenhouse but this was a slab left over from what we called the "Southfork bathroom", an extraordinary creation that we inherited with the house, lined in marble and fitted with gold taps. Our builder managed to get out all the marble intact and most of it was re-used in our kitchen to make worktops and splashbacks. The leftover slab was fitted in the greenhouse. Standing here are various pots of scented-leaved geraniums (cuttings waiting to be potted on), two clay pots of pink-flowered Primula malacoides which is only half-hardy and the big plastic tray that until last week held 32 pots of sweet peas.

That sounds a lot, but the pots are small (5cm) and I sow five seeds in each. When I plant, the whole potful goes in together – nine pots on each wigwam. Fortunately, the sweet peas all went out last week and, for a blessed moment, there is some spare space. But the pelargonium cuttings need to be potted on and soon the cleome will need to be pricked out. So will the Cenolophium denudatum, the umbellifer that Tom Stuart-Smith used so brilliantly in his Chelsea garden last year. Thank heavens the clocks have changed. I need that extra hour of evening light.