Rubus suavissimus Seeds | Sweet-leaf Raspberry
LATIN NAME: Rubus suavissimus
COMMON NAME: Sweet-leaf Raspberry
Plant Profile
A deciduous shrub of the rose family, Sweet-leaf Raspberry grows arching, biennial canes from a woody crown and reaches about two metres where it is happy. The leaves are the reason it is grown: broad, three- to five-lobed, deeply veined, and sweet enough that a single leaf held on the tongue tastes of sugar. White five-petalled flowers open in spring, followed by small red raspberries that are pleasant but seedy.
The species comes from the hills of Guangxi, Guangdong and neighbouring provinces in southern China, where it grows in thickets and along forest margins on acid soils. Local people have picked the leaves for generations and dried them as a sweet tea, which is how the plant first reached growers outside China.
Science & Biotechnology
The sweetness is not sugar. The leaves accumulate rubusoside, a diterpene glycoside of the kaurene type that tastes roughly sixty times sweeter than sucrose, together with smaller amounts of related suaviosides. Because the compound is not a carbohydrate, it has attracted interest as a natural high-intensity sweetener and as a masker for the bitter aftertaste of steviol glycosides.
Biotechnologists have taken the work further. Glucosylation of rubusoside by engineered enzymes improves its solubility and rounds off its liquorice note, and the molecule has been used experimentally as a solubiliser that carries poorly water-soluble drugs into solution. The leaves also contain ellagitannins and flavonoids of the kind common throughout Rubus, which is why the tea is mildly astringent.
Uses & Benefits
In Guangxi the dried leaves are sold as sweet tea and brewed on their own or mixed with ordinary tea to soften the bitterness of herbal decoctions. Home growers use them the same way: two or three fresh leaves sweeten a pot without adding sugar, and the flavour holds up to drying far better than the fruit does.
The berries are edible but small and full of hard seeds, so they are mostly a bonus in late summer. As a garden plant the species works as a hedge-height thicket at the back of a border, where its arching canes and broad leaves make a dense screen, and its spring flowers feed early bees when little else is open.
Growing from Seed
Rubus seed is famously slow. Sow fresh seed in autumn on the surface of an acid compost, cover it with a few millimetres of grit, and leave the pots outdoors through winter so that cold and moisture break the hard coat; germination may not begin until the second spring, and a proportion will wait a year longer.
Where the plant is already growing, layering a cane or lifting a sucker is far quicker, and named stock is usually raised that way. Grow the young plants in a sheltered frame for their first year, then set them out a metre and a half apart in soil with plenty of leaf mould, pH between 5 and 6, and a mulch that keeps the roots cool. Cut the canes that have fruited down to the ground each winter and feed with compost in spring; the species is subtropical, so protect it from hard frost in cooler gardens.
Our Take
A plant grown for its leaves rather than its fruit, and unusual enough to be worth the space. It wants acid soil, some shade from midday sun and a winter mulch, and it will crop for years.
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