Your first line of defence against weeds should be cultural controls; no herbicide programmed will work effectively without good nursery hygiene. Any established weeds around the crop that are allowed to set seed will create huge pressure that can reduce the effectiveness of herbicides. Non-crop areas and paths should be kept clean and free of weeds that could run to seed.
Past work has shown that a little-and-often approach with a weekly hand weeding routine is more effective and requires fewer labour inputs overall compared with a periodic purge; the ultimate aim should be to avoid any weed setting seed. In production areas plug and liners should be kept clean, and remove all weeds prior to potting. Woven plastic bed covers (e.g. MyPex) should be maintained and replaced when required. Beds should be well draining to help kept them clear of liverwort and moss, helping to prevent further weed establishment.
Gravel or sand beds can be sprayed with residual herbicides like Chikara prior to standing crops down on beds. Chikara can give up to five months residual weed control, as a rule of thumb most residual herbicides persist for up to twelve weeks. Plan your irrigation schedule to maintain a dry regimen and avoid excess irrigation to reduce leaching of residual herbicides and reduce the weed pressure for species like willows (Salix) that require damp surfaces to germinate.
Keep old and new stock separate as older stock generally has a heavier weed burden which can easily spread. You may also wish to consider how long you wish to retain old stock as the costs of cleaning up older stock can outweigh the costs of disposal and starting from scratch.
The full technical advice sheet can be downloaded below.