Planting out biennials in September

Planting out Biennials in September

Biennials can be really useful plants to grow because of the time of year they flower. In the lesson 'sowing biennials', we looked at how to sow biennials, and now it's time to go through when and how to plant them out.

Apart from collecting seed, the other main job for September is planting out biennials sown in June or July.

• September is a good time to plant as it's not usually so hot and dry, so less watering will be required than for spring planting.

I'm planting Hesperis matronalis (sweet rocket) and Erysimum mutabile (changeable wallflower).

• These won't look like they're doing much over winter but will be forming a good root system ready to flower next year.
• You can leave them in pots over winter, but if they're hardy, it's a good idea to plant them in the ground and let them get their roots established in their final position.
• Water in and then you can leave them to look after themselves over winter.

I grew the sweet rocket in root trainers for good long root systems and am planting them in groups of three. I'm hoping these will self-seed and I won't have to sow undercover again.

• It's always a good idea to plant in odd-numbered groups (e.g. 3, 5, 7) for a natural look.

Filling a Gap in the Year with Biennials

Biennials are sometimes overlooked because people think they're tricky to sow or don't know when to sow them. However, they're a really useful thing to grow because they flower slightly earlier than the hardy annuals and perennials – usually in May – which creates a foil for late bulbs such as tulips and fills that gap before the main flowering season in your garden.

Plant Directory

Erysimum mutabile

Changeable wallflower

Hardy sub-shrubby perennial

Brassicaceae

Hesperis matronalis

Sweet rocket, dame's violet

Herbaceous hardy biennial or short-lived perennial

Brassicaceae

Tulipa species and cultivars

Tulips

Bulbous hardy perennials

Liliaceae