Growing and Propagation

FEBRUARY 2020: PROPAGATING DAHLIAS, CHRYSANTHEMUMS AND PELARGONIUMS

In February, I start to prepare our Dahlia, Chrystanthemum and Pelargonium stock plants for propagation.

Dahlias

I propagate our Dahlias in two main ways, by divisions and cuttings. Dahlias are incredibly productive plants and are capable of growing from a rooted cutting taken in the spring into a fully flowering plant with a tuber the same year. There’s some debate over which method is better. Really, they serve different purposes.

Cuttings are generally favoured by exhibition growers who like the control that a single stemmed plant grown from a cutting gives them. This helps them on the way to producing perfect blooms that match all the showbench criteria. This is less of an issue for cut flower growers who generally want multiple blooms of smaller size. However, taking cuttings is a great way for us to bulk up stock quickly.

A tuber will send up multiple stems each season. These will branch and branch to produce lots of flowers great for cutting. You do need to keep an eye on the number of stems produced though and remove any more than 4 or 5 so that the plant doesn’t become congested. This is a reason for dividing your tubers. Some growers suggest dividing them down as small as single ‘fingers’ but I find this doesn’t produce good plants in our fairly short growing season. Instead, I prefer to keep the tubers a little larger, say around the size of my palm. I use a saw to divide my dahlia tubers and this year I’m trialing dusting them with cinnamon to combat the grey mould which tends to plague them during overwintering storage.

To take cuttings, I start the dahlias into growth in trays of compost on a heated propagator bench in February and when a new shoot is about 10cm long, I remove it, taking a small sliver of tuber with it. I dib a hole in a 9cm pot of compost and firm the cutting in. I water it and place the pot back on the heat mat. After a couple of weeks, the slice of tuber will have begun to produce roots. When the roots have filled the pot, I pot the dahlia on into a 2L pot and grow it on in a frost free place until it’s ready to plant out in May.

Chrysanthemums and Pelargoniums– we love growing Chrysanths and Pelargoniums and have been working on bulking up our stock for the last couple of years. We do buy new varieties in from Chrysanthemums Direct but we try and propagate from our existing plants too. We only grow these in our polytunnel and they’re in the ground from June to late November.

When they finish flowering, we cut the top growth back until we’re left with a stump about 6 inches high. We lift these plants and put them into a large potting tray with loose compost and keep them dry overwinter.

In the warmth of the polytunnel, the plants begin to make new vegetative growth in February and I take cuttings from them. I do this by cutting the top of a stem about 10-15cm long just above a node where two leaves are growing. This will stimulate the stem that remains on the parent plant to branch and create more stems. I then prepare the cutting by trimming it to just below it’s bottom leaf node and remove the lowest pair of leaves. Hormones that stimulate rooting gather here and so this method is more reliable than just sticking the cutting in a pot with a peg of stem extending below the bottom set of leaves (which is what you’d get if you used it straight as you cut it off the plant). I give rooting an extra boost by dipping the cut end in some hormone rooting gel as well. The cuttings will be placed on a heated bench which also aids root formation. I purchased a thermostatically controlled heat mat from Jungle Seeds and sandwiched it between a polystyrene board and a sheet of capillary matting to create my own heat bench.

Once the cuttings are prepared, I dib 5 of them around the edge of a 9cm pot of compost mixed with perlite and water them in. There is better drainage and airflow at the edge of the pot relative to the centre – which aids good rooting. This method is also more efficient as you can get more cuttings in a pot. It’s really important to try and reduce water loss from the cuttings at this stage. Two ways you can do this are to cut any large leaves in half to reduce the surface area that water can be lost from. This doesn’t seem to harm the cutting. The other way is to keep the cutting covered. Some people use plastic bags but I find this can cause too much moisture to build up and encourages rot. I prefer to cover my heated propagator with a plastic lid. This gives a bit more space around and above the cuttings whilst still keeping the air inside humid. I like to take the lid off for a short time each day to air the cuttings without drying them out.

Once the cuttings have rooted, I pot them up individually into 9cm pots and then repot them again into 2L pots before planting them out in the polytunnel in June.

By doing this, we can hope to have a late summer full of glorious blooms and fabulously fragrant foliage.

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