Weighing up weed control options in CNS

2 November 2016
  • Whastsapp

Autumn, being a slower time of year, provides an ideal opportunity to catch up on maintenance jobs including weeding, removal of debris and replacing old mypex pathways etc.  Here Andy Hodges, Technical Area Sales Manager for the South West, explains how cultural techniques, chemicals and good housekeeping can all play a role in weed control in modern container nursery stock (CNS) production.

Since the advent of CNS, the suppression and control of weeds has been a constant issue for growers.  Recently, with fewer chemicals available, growers have had to look at husbandry and cultural techniques to lessen their impact on the bottom line. 

Weeds allowed to grow and flourish quickly reproduce and spread from pot to pot competing with plants for light, water and nutrition.  While impacting on final plant quality, weeds also increase labour requirements preparing plants for marketing – both negatively impacting on bottom line profitability.

In their war on weeds, growers can employ a range of measures, but these need evaluating before being put into practice to ensure best value is achieved.

Clean Start: young stock, plugs and liners need to be clean, both of weeds and seeds prior to potting. Beds and setting down areas and surrounds also need to be cleaned and kept clean. Bittercress can complete its lifecycle in around a month with the explosive seed heads then being able to distribute hundreds of seeds to about a metre away. Willow and willowherb have wind borne seeds that can easily travel hundreds of metres on the breeze. So problems can arise from both inside and outside the nursery boundary.

Chemical Controls: a few products are still available and some have new labels amending how and where they can be used. It is imperative that label instructions are followed and that only products with current approvals are used.

Dry Tops: some reduced peat and non-peat growing media formulations produce a drier pot surface that is less hospitable for weed germination. This can also be achieved, to a degree, if capillary or sub-irrigation is used with other growing media mixes.  

It is worth noting that changes in growing media formulation should go hand in hand with closer monitoring to ensure appropriate moisture level within the root area of the pot.  A drier surface can lead to a tendency to water when not necessary.  A reliance on sub-irrigation can lead to increased conductivity levels towards the top of the pot and a detrimental effect on root development, particularly with sensitive species. Previously there have been recommendations to ‘flush’ pots from above to reduce this risk.

Bark or Pot Top Mulches: these mulches often dry out more quickly, again giving a less hospitable environment for weeds to establish.  The advantage is that growing media formulations do not need to be radically changed, nor irrigation systems replaced.  However, mulches tend to reduce evaporation from the pot, so again it is necessary to ensure pots are watered appropriately.

Mulches are often introduced at the expense of growing media and this, in turn, can reduce the amount of fertilizer available to the crop.  For example, a layer not much more than 1cm on the top of an average 3lt pot actually removes 10% of the pot volume – this equates to 10% reduction in any controlled release fertilizer available to the crop.

Bark type products can also have a detrimental effect on Nitrogen availability as it is locked up by microbial activity in the process of breaking down the bark.  This then leads to a lower availability of N for the crop.  The upshot is that fertilizer rates and types should be assessed and monitored through the growing season.

So cultural techniques, chemicals and good housekeeping can all play a part in reducing weeds across the nursery but, as with any change, growing techniques and crop monitoring needs to be adapted too.