From Windowsill to Water Butt: Growing Petunia & Coleus from Seed with Rainwater Terrace 🌱

There’s something truly magical about watching a tiny seed transform into a vibrant, thriving plant. Every spring, gardeners across the UK begin this journey indoors  and one of the most accessible ways to get started is by sowing seeds on a sunny windowsill.

At Rainwater Terrace, we know that growing from seed is not only economical and satisfying, but it's also a powerful way to connect with nature and get the most from your garden, however big or small. This year, we started our own Petunia and Coleus seeds on a windowsill, and the results have been nothing short of spectacular.

self watering water butt planter in bright pink clipped on to a water butt with coleus and petunia grown from seed

Why Start Seeds Indoors in Early Spring?

The British climate doesn’t always allow for direct sowing early in the season, especially with tender annuals like Petunia and Coleus, which need warmth and light to germinate. A bright south-facing windowsill becomes the perfect micro-greenhouse a launchpad for your garden.

Starting indoors gives your plants a head start, ensures strong root systems, and gives you the satisfaction of nurturing them from day one. Plus, it’s a fantastic way to make use of time before the risk of frost has passed.

From Seedling to Statement Plant: The Joy of Transplanting

Once established, these vibrant plants are ready to move outdoors into Rainwater Terrace water butts, where they’ll live out the rest of their season  nourished solely by harvested rainwater.

Rainwater Terrace systems are not only functional, sustainable, and UK-made they also offer built-in planters that transform your outdoor space into a flourishing vertical garden. Transplanting into these planters means your seedlings continue to thrive in a water-efficient, pollinator-friendly environment.

Our Petunias burst into colour, trailing beautifully over the edges, while the Coleus adds bold, architectural interest with its deep red leaves edged in vibrant green. Seeing them grow from seed to showstopper has been a highlight of the season.

Rainwater Only, No Tap Required 💧

One of the most satisfying aspects of this process? These plants are sustained entirely by rainwater. No hosepipes. No tap water. Just nature doing what it does best with a little help from clever design.

Using rainwater is not only better for your plants (especially in hard water areas), it’s also better for the planet. With increasing concerns over water usage and climate resilience, installing a rainwater harvesting system like Rainwater Terrace is a small change that makes a big impact.

Tips for Success When Growing from Seed on a Windowsill

  • Use a clean, shallow tray with drainage and a good-quality seed compost

  • Label your trays — it’s easy to forget what’s what!

  • Turn your trays daily to prevent leggy seedlings leaning towards the light

  • Keep soil moist but not soggy

  • Harden off seedlings gradually before transplanting outdoors

Make This Spring Count 🌿

Whether you're growing colourful annuals like Petunias or striking foliage plants like Coleus, the process of growing from seed and seeing your plants thrive outside on a Rainwater Terrace system is one of the most rewarding gardening experiences there is.

Join the movement toward sustainable, rain-fed gardening. Give your plants the best start indoors — and the best home outdoors.

📸 Pictured: Our Coleus and Petunia seedlings thriving on a sunny windowsill before being moved to a Rainwater Terrace planter.

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