From Butterfly Gardening by Jacqui Knight: During their life cycle, Lepidoptera species (butterflies and moth) require nectar-producing plants for adults to feed on as well as host plants where the females lay eggs and caterpillars or larvae will thrive. Flowers are what bring them to your garden; their shape or arrangement, colour and scent all play an important role. Butterflies are attracted to groups or stands of plants that provide a mass of colour. Purple, yellow, pink, white, orange and red flowers all work well but because butterflies are nearsighted, a single plant may not produce enough colour for them to see.
As they need to land in order to feed, the shape and arrangement of the flowers needs to provide a good landing surface. Asters are great in this regard, while salvia and morning glory have tubular flowers with large amounts of nectar. Aster, bee balm, catmint, coneflower, cosmos and dandelion are some great examples. Then there’s honeysuckle, hyssop, lantana, lilac, marigolds and especially good is Mexican sunflower (Tithonia rotundifolia). Some other favourites are phlox, pinks/Sweet William (Dianthus), primrose and sunflowers. And last but not least: verbena, viburnum, yarrow and zinnia. But certainly this list is not complete – just try and get a mix of colour, height, texture and flowering periods.
Check out the Monarch Butterfly New Zealand Trust for lots of wonderful resources.
The last monarchs of each season will overwinter in trees such as in parks and on golf courses and at the end of the winter return to gardens to reproduce, starting the next generation. Trapping wasps helps butterfly populations as wasps take monarch caterpillars as a source of protein for their young.