Cut your rhubarb stalks into smaller pieces/slices.
Place the rhubarb pieces into a large jam pot.
Add sugar and the juice of the lemon. You can add the lemon halves too if you like (extra natural pectin).
3 ½ cups Sugar, 1 Lemon
Mix everything well. At this point, you can choose to leave the ingredients to infuse for a while (i.e., macerate overnight), which might result in a better jam, or you can start to cook the jam straightaway.
Bring your jam to a rolling boil and stir occasionally.
Take down the heat and allow your jam to simmer or cook slowly until it is set.
Check if your jam is set with a candy thermometer, 220 Fahrenheit/ 105 Celsius. Also, do the setting test by dropping some hot jam on an ice-cold plate (which you kept in the freezer previously) and move around the plate to see if the jam is running. The jam is set if it's not moving, if not, cook your jam further and repeat the test until set. *see Notes
Pour jam into clean sterilized jars and fill up to the rim. Close with a lid tight and turn upside down quickly.
Leave the jam like that until it has cooled a bit, and you can turn it back. Don't keep your jam upside down for too long or else you will have unwanted vacuum pockets forming at the jar base.
Label and store in a cool and dry place until further use. Store in the fridge once opened.
Video
Notes
Use garden fresh organic untreated rhubarb if possible.
Do not reduce the sugar in the recipe. Read up in my post why.
Use the lemon juice and lemon halves in your jam. The lemon halves contain plenty of natural pectin, which helps to preserve the jam.
Only use untreated lemon if you use the lemon halves in the jam because of the pesticides that they put on the lemons. You don't want pesticides in your jam.
The cooking time is relative and can be anywhere somewhere between 30 and 90 minutes. Please read up my post for details about setting the temperature and time.
You have to test your jam if it's set, it's a must or else you won't know if your jam will store well at room temperature.
Test with the ice-cold plate. When the plate is ice-cold and some hot cooked jam is dropped on the ice-cold plate, the jam cools down instantly and reveals its true consistency. Hot boiled jam will always appear liquid, and that can be misleading because you will think that it's not set. Jam thickens when it cools down.
I use the ice-cold plate method only at home, but if you are unsure and this is your first jam, get a candy thermometer to help you understand when a jam is set. Generally, it makes sense to use a candy thermometer because of food safety.