Overwintering sweet williams: winter hardiness, care & pruning

Care of the hardy sweet william is easy to manage. If the right location has been chosen, you can look forward to a pretty flowering cottage garden classic.

The nostalgic flowering perennial not only displays attractive flowers, but also spreads the scent of cloves. You can read more interesting facts about the sweet william (Dianthus barbatus) in our general article about this beautiful ornamental perennial. We have summarised here whether sweet williams are hardy and what the optimum care looks like.

Are sweet williams perennial?

As a rule, sweet williams are biennial plants that die back after flowering in the second year of vegetation. Depending on the variety chosen and on very favourable sites, however, the sweet william can also grow perennially. It is also possible for the sweet williams to seed themselves, so that they can be seen in the garden for several years in a row.

Are sweet williams hardy?

Yes, if the sweet william is planted or sown at the right time, it can tolerate temperatures as low as -34 °C. Therefore, special overwintering is not necessary. However, sweet williams in tubs should ideally be wrapped in fleece or jute, as the root space is limited there and frost penetrates the soil faster and deeper than in the grown soil, meaning root damage could occur.

Caring for sweet william

Pretty sweet williams are easy-care flowering perennials. If you offer them the right location, you can look forward to a beautiful display of flowers. Our special article tells you everything you need to know about planting sweet williams.

Watering and fertilising

Sweet william grown outdoors need fresh soil – which means that the water requirement is relatively high. Sweet williams should therefore be watered regularly, unless nature has already taken over. If your garden soil is rather poor in nutrients, we recommend two applications of fertiliser per year – once in spring and once in August. So-called slow-release fertilisers such as our Plantura All Purpose Plant Food are suitable for this purpose. This provides the sweet william with a sufficient amount of nutrients during the growing season. The high proportion of organic parent materials also ensures a robust root system and active soil life.

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The same applies to sweet williams in planters – the substrate should always be kept moist. If the sweet william has been planted in a fresh and high-quality planting substrate that has already been pre-fertilised, then further fertilisation is not necessary in the same year.

Tip: Unfortunately, the pretty sweet william is often attacked by pests or fungi. These include, for example, spider mites (Tetranychidae), thrips (Thysanoptera) and aphids (Aphidoidea). Rust (Pucciniales) can also become a nuisance, depending on the resistance of the respective sweet william variety. You can recognise an infestation by pale rust pustules on the outside and elongated brown pustules on the inside of the foliage. Fusarium wilt also occurs in sweet william. Infested specimens wilt and the leaves change colour. The fusarium fungus grows in the transport system of the sweet william, meaning transport is no longer possible. As the spores spread via water, wind and clinging to materials, the plant and soil must be disposed of and everything thoroughly washed off hot.

Cutting sweet williams

In general, pruning of the one to two year old perennial is not necessary. However, to extend the flowering period, it is advisable to remove the faded flowerheads. Otherwise, sweet williams are pruning tolerant. This means you can use them as cut flowers without harming the perennial.

Tip: If the faded shoots are left standing, seeds will form later, allowing sweet williams to reproduce themselves. With a bit of luck, you can look forward to the next generation of the sweet william next year.

The bee-friendly littleleaf lilac (Syringa microphylla) also produces beautiful flowers, which it puts out until October. Its compact growth habit also makes it wonderfully suitable for small gardens.