NDS classification:- Group 9. Semi cactus dahlias have fully double blooms,the ray florets are usually pointed,and revolute for more than 25% of their length and less than 50% of their length (longitudinal axis),broad at the base and either straight or incurving.The blooms are usually between 102mm(4") and 152mm(6") diameter.
Spray tubers with a fungicide around week 4-5 then place on hot bed where the temperature is set to give a soil temperature of around 18-20*C.
Normally it takes around two to three weeks for shoots to appear and a further four or five weeks to produce shoots that are suitable for cuttings.
Take cuttings approx 50-60mm long around week 11-15. Remove bottom leaves and insert approx 30-40mm apart into a mixture of 50-50 (by volume) of peat and grit sand (the use of rooting powder/liquid is optional).
Water cuttings in with a fungicide solution.
Mist spray (daily) with a fungicide solution to prevent dehydration and damping off.
Dependant upon available light the cuttings should take around two to three weeks to root. When the tip of the cutting looks a healthy shade of green it has rooted.
Prick out into 70mm pots filled with potting compost, place them in a well lit position but out of direct sunlight.
Circa week 18 place cuttings in a cold frame to harden off.
Plant out in pre-prepared beds around week 22, or when all fear of frost has gone.
Circa week 26/27 remove the growing tip of each plant to encourage bushier growth.
Circa week 28/30 top dress the beds with a fertiliser such as Fish, Blood & Bone.
It can be advantageous at this time to mulch the beds with straw or spent mushroom compost to conserve moisture.
Give plants a high potash feed around week 30/32 to stiffen the stems and enhance the colour of the blooms.
Dahlias require copious amounts of water during their growing season.
End of season preparations
Cut down the plants after they have been blackened by frost to around 150mm above ground level, carefully lift the tubers, remove the bulk of the soil from around them and leave them upside down in a dry frost free area.
When they are quite dry remove the dried soil and trim off the thin water roots and any damaged parts of the tuber and store them in a cool frost free area until required.
Check periodically for signs of decay, remove any rot that may appear.