Fertilisers: Start applying topdressing to stock plants and trees. Use a horticultural grade 1:1:1 fertiliser where the source of potash is potassium sulphate rather than potassium chloride.
Some ornamentals can be chloride-sensitive, resulting in leaf damage on some soils. On over-wintered container plants that do not have the longer-lasting CRF products, consider a granular product such as Sincron, Triabon or Floranid Permanent as they break down and leave a better visual look to the pot surface. CRF plugs pushed into the growing media work well but with some rootzones this can be quite difficult to insert. Application rates vary according to the product but as a guide use 1g/litre of compost or one 5g plug per two-litre pot. Remember that applications of topdressing have been shown to destroy the herbicide layer at the point of application; you may need to consider a herbicide application about three weeks after a topdressing application to avoid unwanted weed growth.
Slugs: As the leaves of Hosta increase in size, that full pot of foliage makes a wonderful meal for slugs, which can quickly ruin a saleable plant. Ferramol is much safer for the environment and does not look as unsightly as some of the older pellets that have been present for some time. Decoy Wetex, while a large pellet, does retain effectiveness even in an overhead irrigation regime. HDC factsheet 02/09 for the FV 225 project provides useful information on the control of slugs in field situations, which may be helpful for bareroot herbaceous perennials.
Lavender: The common shab disease or Phoma is now showing up on lavender plants as they begin to grow away. If you see branches dying back from the base, drench with Scotts Octave or Cercobin WG. Dying from the tip downwards is evidence of Botrytis and needs a Rovral WG, Switch, Frupica SC or Amistar spray to the growing points.
Clematis: Drench newly potted Clematis with Aliette 80WG followed by a spray of Scotts Octave as a precaution against Clematis wilt.
Downy mildew: A reminder that Osteospermum, pansy, Digitalis, Geum, other susceptible herbaceous perennials and Hebe will still be vulnerable to downy mildew during March as the temperature increases. Spray with Fubol Gold WG (SOLA required) or Aliette 80 WG if the tell-tale yellowing on the top surface and angular darker patterns is seen on leaves. You may like to add a wetting agent to get good under-leaf coverage.