Invasive Species Cookbook – Japanese Knotweed Jam

This week (13th – 17th May 2019) is invasive species week and Japanese Knotweed is probably the most famous of the species to invade the UK. The legendary concrete-smashing, tarmac-raising plant is feared by the construction industry and gardeners alike.

Japanese Knotweed first arrive at the Royal Botanical Gardens Kew in August 1850 in an unsolicited parcel of plants. The plants were then sold by a large number of commercial nursery gardens around the country. The plant is a herbaceous perennial, with stems typically about 2m tall and an extensive system of rhizomes. It has large, roughly triangular leaves with truncate (not cordate (heart-shaped)) bases.

The sharing of cuttings and the discarding of unwanted rhizomes established the primary pattern of distribution for Japanese Knotweed around the UK followed by natural spreading along watercourses, and artificially where soil containing rhizomes was moved above in road building and construction schemes.

Japanese knotweed is an invasive non-native weed in urban areas where it is considered a nuisance in property development as the rhizomes are known to exploit weaknesses in structures and can come up through gaps in walls, concreted and flooring.

Japanese knotweed Jam

In the UK Japanese knotweed has no natural predators however it has one weakness - it is edible at certain times of the year! As with Rhubarb the shoots in the spring are tender enough to eat but they have to be gathered before the stems become hard and woody. The ideal time to eat knotweed is mid-April to May.

Japanese knotweed is a source of Vitamins A and C and contains potassium, zinc, phosphorus and manganese and has been used for treating many ailments, such as respiratory infections. It’s a free-range food and eating it can slow the spread of this plant.

To forage the plant ensure you have landowners permission and a stand of knotweed that hasn’t been sprayed. The leaves and tough stems are not edible, so you’ll need to discard these carefully. We put all our leftover bits in a baking try and baked for 30 mins until the stems were mushy and the leaves were baked crisp.

The legal bit - There is no legal obligation to remove or treat knotweed as long as you're not encouraging or allowing the growth on to adjacent land. As of schedule 9 of the 'Wildlife and Countryside Act 1981', you must not plant or cause to grow Japanese Knotweed in the wild.

Our recipe for Japanese knotweed jam is based on rhubarb and ginger jam as the two plants are similar in taste and texture. Get foraging and enjoy!!


·         1kg Japanese Knotweed Stems. Leaves removed (top up with rhubarb is you can’t forage enough)

·         1kg jam sugar (which has added pectin)

·         zest and juice 1 lemon

·         50g stem or crystallised ginger

·         4cm piece of Ginger, chopped


·         Wash the Japanese Knotweed under cold running water and slice into 2cm pieces. Tip into a large ceramic or plastic bowl and add the jam sugar, lemon zest and juice, and chopped stem ginger. Finely grate the peeled ginger directly over the knoweed.

·         Stir the mixture thoroughly, cover loosely with cling film and leave to one side for about 2 hrs to allow the sugar to dissolve into the juices. You may need to stir the mixture occasionally to encourage this process along.

·         Put the knotweed and all the sugary juices into a preserving pan and set over a medium heat. Stir until the sugar has completely dissolved and bring to the boil. Continue to cook at a fairly swift pace until the mixture is really tender and the jam has reached setting point (about 10-15 mins).

·         To test for a set, drop ½ tsp of the jam onto a cold saucer, leave it for 30 secs, then gently push it with the tip of your finger. If the jam wrinkles the setting point has been reached. If not, continue to cook for a further couple of minutes and test again.

- Remove the pan from the heat and leave to one side for 2-3 mins before pouring into sterilised jars.

Japanese Knotweed jam

Japanese Knotweed jam

Boiling Jam

Boiling Jam

Stewart Parsons