I have never grown Weigela from seed but I do find the Thompson and Morgan seed raising guide very helpful. It is no longer in print but still available online. Just Google. - Ian.

Weigela grow so easily from cuttings, I have never tried seeds. T&M does not list Weigela on their seed list. Al

The Clothiers site says only to direct sow weigela seed. Druse (Making More Plants) states simply 'seed - outdoors in Spring' so from that I'll assume they germinate at coolish temps but don't need a prolonged moist chill.

Plant World Devon (and they do give very good sowing suggestions with their seed) says "Sowing advice: COVER WITH COMPOST OR GRIT 2mm. DEEP. SOW ANY TIME IN A COOL BRIGHT SPOT OUTSIDE. MAY BE SLOW TO GERMINATE. GROW ON INDIVIDUAL SEEDLINGS IN 3" POTS. PLANT OUT IN A WELL-DRAINED SHELTERED POSITION".

I wouldn't usually quote a seed vendor for growing instructions, but I'm not finding a lot of propagation info for weigela seed (and probably because they are easier, faster, from cuttings) in my usual sources and I do have plenty of experience with seed from Plant World - if that helps you at all.