Until the past two winters, gardeners were pushing boundaries with frost-tender plants, including many Mediterranean and desert varieties, and getting away with planting them outside.
But now that many have suffered plant casualties, the risks no longer seem worth taking.
Agave is a good example. Gardeners with sheltered spaces, particularly in the South West, loved to use their structural forms to grace outside areas. Agave americana Award of Garden Merit (AGM) is hardy to -9 degsC if kept dry, and this was a popular choice. But now nurseries are reducing their stock of larger specimens of these and are sticking to varieties that can be grown in the house, the conservatory or the greenhouse.
Agave are in the Asparagus family and are known, rather confusingly, as the century plant in reference to the length of time it takes for them to flower - it can take 30 to 40 years for larger species, although much less for others. The thing to note about many Agave species is that they are monocarpic, meaning that once they have flowered the plant withers and dies. In the wild they will usually have produced seedlings or offsets, but in cultivation such a loss might be a bit disconcerting.
Around 300 species are cultivated, largely because of their highly structural foliage that forms a rosette of succulent, sword-shaped leaves. These leaves are often edged with spines and are very long-lived, making them an evergreen presence. They can grow very large indeed - up to 7m in diameter.
They suit pot cultivation because it means they can be out on the patio during the summer, but brought in over winter. Despite their potential size, Agave do not mind being pot-bound. A good mix to use is 50:50 potting compost and horticultural grit. Top-dressing them with gravel also sets them off nicely. Do not use saucers under the pots because they must not sit in water.
For those gardeners who have a site that is virtually frost-free and sheltered, some species can be risked outside if they are planted in a highly-drained medium. This prevents them getting their roots wet in winter, which would be their downfall. A bed can be constructed specifically for arid plants, consisting of rubble covered with finer stone and grit. It is best to plant out more mature plants because small plants are not likely to survive a cold winter.
Indoor and outdoor plants are boosted by a half-strength high-nitrogen feed in early summer and a half-strength low-nitrogen feed in early autumn. Be sparing with water, keeping the plants dry over winter
Plants under glass should be watered a little more in the spring and autumn, allowing them to dry out between waterings. Outdoor plants only need water during dry spells, with at least a two-week gap.
What the specialists say
- Steve Murnan, owner, Select Succulents, Bristol
"Agave is visually stunning, structurally bold, highly variable and an undemanding genus. They look great in pots on a patio or planted out in an arid bed. Larger specimens create a real wow factor in any garden. Combine this with good pest resistance and low maintenance and you have a winner.
"Some of the Japanese-developed varieties are superb, with their variegated leaves and burgundy spines. I really like A. 'Kissho Kan' and A. schidigera 'Shira ito no Ohi'. Both are ideal pot plants for the conservatory or greenhouse.
"But I have two favourites. One is A. victoriae-reginae 'Golden Princess', suited to the unheated conservatory or greenhouse. Plants grow to 1.8m and have golden-yellow variegation to the edge of the leaf. It's a compact and marked plant that is rather uncommon in cultivation.
"For outdoor display in pots or arid beds, my favourite would be A.americana 'Opal'. This is a variegated variety that will develop into a 3m tall by 4m wide plant given time, but can also withstand temperatures as low as -9 degsC. This has to be the ultimate impact plant for our climate and A.americana AGM has a proven track record in outdoor cultivation here in the UK going back hundreds of years, especially in the South West."
- Neville Bell, owner, Glenhirst Cactus Nursery, Lincolnshire
"I think Agave is popular because there are attractive species to suit most conditions and situations. One that stands out for me is A. deserti. It has a grey-white bloom on the leaves that is very attractive. I also like A. utahensis AGM and its varieties - they all have a neat, compact habit. My favourite is A. victoria-reginae AGM. Its symmetry of shape is beautiful.
"My growing tips for Agave are to give them plenty of space. Water and feed them well in the spring and summer. They experience no real problems. They should be kept dry over winter, and given very little water."
- Paul Higginson, grower, North South Succulents, North Shropshire
"It's an incredibly diverse genus. The variegated types are always very popular, as are those with attractive symmetry. I particularly like the mediopicta forms (variegated colour on mid-leaf). My favourite species is A. hurteri for its simplicity of form.
"Agave shouldn't be overwatered, especially going into a UK winter. Give them a little water once in six weeks when temperatures allow and then build the amount up in early spring as this can help avoid root loss and scorching."
- Pete Wood, planteria manager, Golden Acres West Parley garden centre, Dorset
"Customers would come to us for A. americana AGM, A. parryi and A. montana. But the past few winters have meant that fewer people are risking them outside. Nurseries are growing them in smaller numbers so the stock isn't available. We always sell Agave as houseplants though - mainly A. ferox."
Species and varieties
- A. americana AGM (H1) is a frost-tender species, with architectural grey leaves and spines. Only recommended for milder sites outdoors, although hardy to -9 degsC if kept very dry. Height: 2m. Spread: 3m.
- A. americana 'Marginata' AGM (H3-4) is a variegated form with grey-green yellow-margined leaves. Relatively hardy if kept dry over winter. Height: 2m. Spread: 3m.
- A. americana 'Mediopicta Alba' AGM (H1) has leaves with a bold creamy-white, central stripe and blue green margins. It may be grown outside in summer, but is better brought indoors during the winter. Height: 1.5m. Spread: 1m.
- A. americana 'Variegata' AGM (H1) has spiny, yellow or white margined leaves. It tolerates down to -5 degsC if protected from wet. Height: 2m.
- A. attenuata has soft, grey-green foliage. It is best grown outside in a pot and brought inside over winter. Smaller plants can be grown as houseplants. Height: 1m. Spread: 2m.
- A. chrysantha is a large, hardy species with inward-facing spines and a sharp tip. The leaves are grey-green and can grow up to 1m long. Height and spread: 2m.
- A. ferox can become huge and suits planting out and large containers. It has wide, dark green leaves with serrated edges and spiky tips. Proven hardy to -8 degsC if kept dry, but also offered as a houseplant.
- A. filifera AGM (H1) makes a great house or patio plant. It has fine white fibres down the margins of the leaves. Height: 50cm. Spread: 65cm.
- A. havardiana can provide great structure in an arid bed with its wide fleshy leaves with curly spikes. Height and spread: 80cm.
- A. parryi is slow-growing and forms a neat blue-grey rosette. It is probably the hardiest Agave grown outside in the UK, but must be in well-drained soil. Also great in containers. Height: 45cm. Spread: 70cm.
- A. parviflora AGM (H1) is a compact, short-leaved plant with curly white fibres emerging from leaf margins. Hardy to -5 degsC if kept very dry.
- A. salmiana var. ferox grows to 3m tall once established, with large green lance-shaped leaves with spines.
- A. victoria-reginae AGM (H1) forms dense, globose rosettes with short thick green leaves edged with white. Black-edged forms are also available. Makes an excellent houseplant. Height and spread: 50cm.
Thank you to Floramedia, which supplied the images for this article from it's photo library www.floramedia-picture-library.com