A Tree Lupin in the garden

Tree Lupin, Lupinus arboreus.

Our Tree Lupin, Lupinus arboreus, is in flower in the back garden. Its pale yellow flowers are a contrast to other flowers at the moment, such as the pink and white of Sweet Rocket (Hesperis matronalis) and the red-flowered climbing Rose ‘Allen Chandler’. There is a blue shed behind the Tree Lupin, giving more contrast, and a Golden Hop (Humulus lupulus ‘Aureus’) trailing up a support next to it, whose leaves match the colours of the lupin. The lupin’s leaves are silky, grey-green and palmately-lobed, contrasting in form, as well as staying green throughout the winter.

Tree Lupin, Lupinus arboreus, with Golden Hop (Humulus lupulus ‘Aureus’)

Our Tree Lupin really belongs in the garden and it’s easy to forget that I only planted it three years ago. I bought it as a potted plant from Hethersett Plant Fair (note 1) at the beginning of May 2022 and planted it a couple of days later.

The Tree Lupin isn’t a tree, and never will be. It’s a short-lived shrub, lasting up to seven years. Our plant is now 1.5 metres (five feet) tall and about 1.2 metres (four feet across). The RHS website says that plants can grow to 1.5 – 2.5 metres tall with a similar spread. Wikipedia gives a maximum height of 2 metres (7 feet) tall in sheltered positions, but more typically 1 – 1.5 metres (3 – 5 feet) tall. I prune our plant whenever it grows across a path and it doesn’t seem to mind.

Our garden soil is very well-drained sandy loam and this is just what Tree Lupins prefer (note 2).

Tree Lupins in the wild

The Tree Lupin is a native of North America, particularly parts of California. It has been introduced into other parts of the world, including other parts of North America (British Columbia, Oregon and Washington), South America (Argentina, Chile and Falkland Islands), Australia (Tasmania and Victoria) and New Zealand (North and South Islands). In Europe it now grows in Great Britain and Ireland.

Lupinus arboreus is a neophyte and was introduced into the British Isles in 1793, when it was treated as a tender plant requiring greenhouse protection. It was first recorded in the wild in 1926 at Freshfield in South Lancashire. It is now well established, with a largely coastal distribution. But there are inland records too: Tree Lupins grow happily on well-drained soils and are given hardiness rating “H4” by the Royal Horticultural Society (“hardy through most of the UK (-10C to -5C)”).

Tree Lupins grow well in sand dunes (where they are widely planted) and on Cornish china-clay tips, as well as on roadsides, railway banks and on waste ground.

Distribution of Tree Lupin, Lupinus arboreus. Image from 2020 BSBI Plant Atlas.

Tree Lupin flowers smell faintly of honey.

The Wild Flower Finder website has photographs of Tree Lupin flowers at different stages of development. Like our plant, the flowers are yellow but the First Nature website has a photograph of a flower with a tinge of mauve. Mike Crew’s Flora of East Anglia website has pictures of white and blue flowers as well as the more usual yellow. Flowering is usually from June until August but it is a bit earlier this year after the sunny and very dry spring.

Although each Tree Lupin plant is short-lived, the plants persist by seeding around. They produce large amounts of long-lived seeds in stiff, hairy pods.

Tree Lupin are members of the Pea family, the Fabaceae (formerly Leguminosae). They have a similar  shrubby, short-lived growth habit to our native Common Gorse (Ulex europaeus) and Common Broom (Cytisus scoparius).

Members of the Fabaceae have nitrogen-fixing bacteria in root nodules (in Tree Lupin, from the genus Bradyrhizobium) . These capture nitrogen from the atmosphere and convert it into ammonia (NH₃) or ammonium ions (NH₄⁺), which the plant can use to grow. This is especially useful in poor soils.

Other Lupins

There are over 199 species of Lupinus worldwide. Most are from North and South America, with some from North Africa and the Mediterranean region.

Mike Crew’s Flora of East Anglia website gives details of two other species of lupin found in our region of Britain:

Narrow-leaved Lupin, Lupinus angustifolius

The Narrow-leaved Lupin, Lupinus angustifolius, is an annual species with very narrow leaflets and blue flowers. It comes from southern Europe and has sometimes been used as a fodder crop. Like other lupins the seeds contain bitter-tasting toxic alkaloids but there is some natural variation and plant breeders have produced low-alkaloid, edible forms (note 3).

Russell Lupins, Lupinus x regalis

Most of us are familiar with Russell Lupins, Lupinus x regalis, which are the common garden Lupin, available in many colours. They are hybrids between the Tree Lupin and the Large-leaved Lupin, Lupinus polyphyllus. They are herbaceous perennials and have inherited this growth habit from the Large-flowered Lupin and a branching habit from the Tree Lupin.

Russell Lupins are often referred to as “”Russell Hybrids”. They grow to 1.5m (five feet) in height and have flowers in many shades of yellow, pink, red, blue, purple or white. Bicoloured flowers are common. Although both parents are from North America, Russell Hybrids were developed on two allotments in York by the horticulturalist George Russell in the first half of the 20th century (note 4).

Pink Russell Lupins, Lupinus x regalis, at Kentwell Hall, Long Melford, 21st May 2025.

We don’t grow Russell Lupins in our current garden, though they would probably do well. Our previous garden in Norwich was full of Garden Snails (Cornu aspersum). These took a particular delight in eating Lupin leaves, tearing them apart with relish and small plants disappeared overnight. Snails weren’t a problem when I lived near Aberdeen and we had many fine Russell Lupins in the garden.

Russell Lupins in a garden near Nethybridge in Scotland, June 2024.

Tree Lupins and wildlife

Lupin flowers are popular with bumblebees and in the last few days worker Buff-tailed Bumblebees (Bombus terrestris) have been visiting the flowers, starting at the bottom of a flower spike and working their way up towards the top.

Bombus terrestris worker visiting Tree Lupin flowers.

On a sunny day the mirid bug Closterotomus trivialis uses our Tree Lupin leaves as a place to sunbathe and, presumably feed. (The bug, a recent arrival from the Mediterranean, is polyphagous, that is, it feeds on a wide range of plants.)

Mirid bug Closterotomus trivialis on a Tree Lupin leaf.

Perhaps less welcome is the Lupin Aphid, Macrosiphum albifrons, which can be found on various species of lupins, including Tree Lupin and Russell Lupins.

Lupin Aphids. Photograph by Vanna Bartlett.

We haven’t found any (yet) this year but in previous years we have had high numbers on our Tree Lupin. We let nature take its course, glad of another species of insect in the garden, but the aphids can sometimes cause serious damage and plants that have been badly debilitated by the aphids can die. The excellent Influential Points website has more details and photographs.

Happy memories!

I have several happy memories of lupins.

Firstly, growing Russell Lupins with my Mum’s help when I was four or five years old. We forgot to water the seedlings but they survived and were the first plants I grew in what started off as a sandpit for me to play in and ended up as my very first garden.

Russell Lupins also remind me of our garden near Aberdeen when we moved to Scotland. I find the smell of Russell Lupin flowers a little peppery and we once made the mistake of picking some of the flowers for a vase. They were soon banished outside; never again.

My first definite memories of Tree Lupins is from the Suffolk coast near the Minsmere RSPB reserve, seen when walking and birdwatching in the area.

More recently, I think of Tree Lupins on the banks of the railway cutting at Felmingham in North Norfolk, where Small-flowered Catchfly grows. The soil is very sandy, which provides good winter drainage, and disturbed, which allows both the catchfly (an annual) and the Tree Lupin to spread by seed.

Finally, earlier this month I went to see the fabulous Oysterband perform at Folk On The Pier in Cromer, on the North Norfolk coast. We walked along the cliff tops from Overstrand to Cromer and, as we started to descend into Cromer there were some Tree Lupins in flower here too. It was a great day out: sunshine, the coconut smell of Common Gorse, good company, tea and cake, fish and chips, a superb concert …and lupins. Perfection!

Tree Lupin in our garden, earlier today.

Notes

Note 1 – Hethersett Plant Fairs are organised by Norfolk Plant Heritage and there are two a year. The fairs are held at Hethersett Village Hall (a few miles west of Norwich) in early May and late August.

Note 2 – The Gardeners World website recommends growing Tree Lupins in full sun, in rich soil (“Acidic / Chalky / Alkaline / Clay / Heavy / Moist / Well Drained / Light / Sandy”). In our north-facing garden our Tree Lupin is in sun for much of the day in summer, but doesn’t receive much direct sunshine in winter.

Note 3 – Several years ago I grew two varieties of Narrow-leaved Lupin, on the allotment, while taking part in a Garden Organic members’ experiment to test the palatability of lupins bred for human consumption.

It was a very dry summer but in spite of this the plants cropped well. But the seeds were very hard and dry and I had to boil freshly collected seeds for an hour or more to soften them. I then discovered that the taste was still too bitter for my liking. I didn’t try them a second time.

I let a few of the plants self-seed and they were very attractive but they eventually died out.

Note 4 – You can read more about George Russell and his lupins online: “The man who made lupins his life” (Yorkshire Post 2004) and “The Lupin Man of York – George Russell” (God’s Own County blog).

At their peak there were 152 named varieties but in the years after Russell’s death many were lost to Cucumber mosaic virus (CMV) or allowed to self-sow, reverting to their original colours.