The best time to put plants into the pond is April or early May. Plant straight into the layer of soil. Choose native species. Firstly you will need submerged 'oxygenators'. You can buy these from garden centres but it is far cheaper to beg some from another pond owner (don't take any from a wild pond). Just two or three small clumps, anchored down with a half-brick, will soon start growing well. Milfoil and water starwort are good native 'oxygenators'; try to avoid Canadian pondweed - it grows like wildfire and quickly takes over a pond!
Other plants to introduce are those which have their roots in the bottom and their leaves and flowers floating on the surface e.g. water lilies.
In the shallows, emergents may be grown in the different depths; these are valuable for sheltering wildlife as well as looking attractive.
Below are some tables of native plants suitable for a garden pond:-
Submerged Aquatic Plants
These provide oxygen and shelter for pond animals
Spiked water milfoil Myriophyllum spicatum
Hornwort Ceratophyllum demersum
Curled pondweed Potamogeton crispus
Water starwort Callitriche Spp
Bottom - Rooted & Floating-Leaved Plants
White water lily Nymphaea alba
Yellow water lily Nymphaea lutea
Fringed water lily Nymphoides peltata
Broad-leaved pond weed Potamogeton natans
Amphibious bistort Polygonum amphibium
Water crowfoot Ranunculus aquatilis
(only suitable for large ponds)
Emergent Plants - Deeper Water
These are bottom-rooted; the leaves & flower stems grow above the water surface
Bog bean Menyanthes trifoliata
Flowering rush Butomus umbellatus
Burr reed Spargahium erectum
Lesser reedmace Typha latifolia
Emergent Plants - Marsh Zone
These can cope with water up to about 150 mm deep
Yellow iris ....... Iris pseudacorus
Water mint Mentha aquatica
Water plantain Alisma plantago - aquatica
Brooklime Veronica beccabunga
Water forget-me-not Myosotis scorpioides
Lesser spearwort Ranunculus flammula
Mud sedge. Carex limosa
Marsh Plants - Bankside Zone
These like damp places but are not true water plants
Codlins and cream or Hairy willowherb Epilobium hirsutum
Marsh marigold Caltha palustris
Marsh woundwort Stachys palustris
Bugle Ajuga reptans
Creeping jenny Lysimahia nummularia
Purple loosestrife Lythrum salicaria
Meadowsweet Filipendula ulmaria
Salad burnet Sanguisorba minor
Ragged robin Lychnis flos-cuculi
Meadow buttercup Ranunculus acris
Hemp agrimony Eupatorium cannabinum