Guy Barter and Terry Walton answer all your gardening questions
Jonathon Thomson, by email
Guy says: Boat gardens are a bit like roof gardens but without the damaging winds and close to an inexhaustible supply of water. Weight might be an issue so use lightweight potting compost with plenty of Perlite to avoid your vessel turning turtle.
It's probably best to use containers rather than creating beds on your decks. Container plants aren't technically low maintenance as they need regular watering and feeding but by using larger pots and perhaps a solar powered pump to feed water into a drip irrigation system, the workload can be kept to manageable levels.
Trees might seem an odd choice but birch and robinia will cast a light shade and, being quite geometric in outline, might remind people of sails if they're suitably positioned. They'll need to be in quite large containers that will have to be lashed down, but ropes and knots are in the water-going tradition, they might even be called rigging.
For an even more geometric shape consider a wedding cake plant, Cornus controversa, in its green or variegated form, that has markedly layered branches.
Water releases a latent heat as freezing point is approached. This heat can fend off the worst chills so a 'floating jungle’ garden is feasible using exotically-leafed plants such as strap-leaved phormiums, cordylines and the silvery Astelia chathamica. Hardy palms are also suitable. Fatsia and its relative, fatshedera, provide big lush leaves, melianthus has striking ladder-like foliage, plus in the south at least, bananas and canna can be considered, both of which will need to be wrapped for winter. Hardy alternatives include ferns and hostas.
Woody plants include pittosporum, which has green, black and variegated foliage, and loquat, both of which respond well to cutting back if they begin to impede navigation. For flowers abutilon, alstroemeria, dahlias and ligularia are worth considering. Dahlias will need to be wrapped for winter but ligularia, a martyr to slugs, might find refuge from its slimy foes.
Finally, consider a watery theme using plants that look like those of river banks and wetlands, perhaps using tall grasses to echo sedges and bulrushes, using coloured stem willows to reflect the willows that line river banks. Other waterside-like plants might include Iris sibirica, rodgersia with huge, hand-like leaves, and the spreading chameleon plant or Houttuynia cordata ‘Chameleon’.
Some plants that like wet feet might relish growing in hanging baskets slung over the side. Iris ensata from Japan, native kingcups and bogbeans, and the richly crimson Lobelia cardinalis should make passers-by take notice.