Window boxes have so little room for compost, making it difficult to maintain good growing conditions for permanent plantings. However, plants like ivies, conifers and heathers will do well for a while, but not really for a few years in the same container.
(It is far better to plant the window box up just once each year – maybe twice if you want a good deal of colour throughout the year.)
Even by having one conifer in a box long term, you can run into problems, for the root system will soon take over all available space. This will make it almost impossible to add extra plants throughout the year, or to maintain healthy plants with the others in the same box.
The obvious way to overcome this is to have the largest possible window box - with the least number of plants. This goes against practicalities with a narrow window ledge - and against human nature to 'skimp' on planting.
If you are looking down or out onto your window box - rather than wanting to view it from street level - your choice of permanent plants increases hugely, for there are a wide selection of 'alpine or rockery' plants, together with heathers and many small evergreen perennials - such as the Sedums and Sempervivums – amd Heathers..
The most successful permanent plantings will be with those plants that are not too bothered about depth of soil. Many of the groundcover plants fall into this category, and will include items such as the perennial Ajugas (Bugle), and for a little bit of height, the evergreen Bergenia. Dwarf or procumbent Hebes will also do well – especially the silver leaves Hebe pinguifolia ‘Pagei’ or Hebe albicans – which will also flower early summer with white blooms.
Other hebes that would be suitable are the whipcord types such as Hebe ochracea ‘James Stiring’ with its sulphur yellow slender twigs. One of my own favourite dwarf or procumbent shrubs would be the prostrate form of Rosemary - Rosmarinus lavendloides.
Any of the silver foliaged plants - dependent upon size - will suit, and in particular the Helichrysum and Artemisia groups.
Almost any of the smaller herbs will grow well, and may even do better than in the garden, for many are happier with shallow soil and dry conditions.
It is a far better idea to use small shrub-like plants, which can then be moved to a more suitable position in the garden after several months. The winter months can be quite colourful if you use winter flowering heathers, Aucubas, Ivies, winter cherries, winter flowering pansies, primroses and the like – under-planted with miniature narcissi (daffodils) for the burst of colour in the early spring.
Change the compost in the spring, and look out for the larvae of the vine weevil beetle – especially if some of the plants seem a bit lifeless!
One group of plants that can do well as permanent planting in window boxes, are herbs. Make sure that you use a few evergreen herb types like the Thymes and ornamental sages!