Quote from Lee Garden Ninja on 8th September 2025, 3:31 pm

Hi @catchaplin

Excellent question about your border planting scheme! You've chosen some absolutely cracking plants for a sunny, well-drained spot, and I can already picture how lovely this is going to look once established. However, let me share some insights that might just transform your planting approach and give you much better long-term results.

The Hard Truth About Your 45cm Border

First, let's address the elephant in the room: your 45cm deep border. Whilst I completely understand the constraints you're working with, I have to be honest with you.  This falls squarely into what I call the "skinny border trap" that I discuss in my 'Garden Design Fails' article.

At just 45cm deep, you're essentially creating what amounts to a single-row planting scheme, which is one of the most common design mistakes I see time and again.

The issue with such shallow borders isn't just aesthetic, though that's certainly part of it. When you can only fit one layer of plants, you end up with a somewhat regimented look that lacks the natural, flowing appearance that makes borders truly captivating. More practically, you'll struggle to create the layered planting effect that gives borders their depth and seasonal interest. Your plants will all be competing for the same soil space, light levels, and growing conditions, which can lead to some thriving whilst others struggle.

Why 80-100cm Changes Everything

If there's any possibility of extending your border to 80-100cm deep, the transformation would be remarkable. This depth allows you to create what I call the "three-layer effect" that I advocate throughout my planting guides. You can feature your low-edging plants, such as the Erigeron, at the front, your mid-height perennials, like the Salvias and Lavender, in the middle section, and leave space for taller architectural plants or even small shrubs at the back. This layering is what creates those gorgeous, professional-looking borders that seem to flow and change as you move past them.

A deeper border also provides you with much more flexibility in plant positioning. You can break the "rules" occasionally by bringing a taller plant forward to create visual interest, or by creating drifts that weave through the different height zones. The extra soil volume allows for better root development, reduced competition between plants, and ultimately healthier growth across your entire scheme.

Plant Spacing: The Magic Numbers

Now, for the numbers you're after!

The general rule I work with for herbaceous perennials in 2-litre pots is approximately 4 plants per square metre. This gives each plant enough space to develop properly whilst ensuring you achieve good coverage relatively quickly. Your 540cm x 45cm border gives you roughly 2.4 square metres, so you're looking at about 9-10 plants total if you stick with this spacing throughout. However, do double check the overall growth and spread of each plant. For more competitive plants you may need to dial this down to 3 per m2 or increase it for smaller specimen plants.

However, plant spacing isn't quite that simple, because different species have vastly different spreading habits. Erigeron Karvinskianus is a particularly vigorous spreader, ultimately reaching 60-90cm across when fully mature. For this plant specifically, you'd want to space them roughly 30-40cm apart, which in your 540cm length would give you about 13-18 plants if you planted the entire front edge with just Erigeron.

But here's where it gets interesting with your mixed planting scheme. If you're interspersing with Lamb's Ear (Stachys byzantina), you need to consider that this also spreads quite enthusiastically, typically reaching 45-60cm across. Both of these are what I call "confident spreaders," meaning they'll naturally fill gaps and merge together over time.

Planting in Drifts

Rather than calculating exact numbers, I'd suggest thinking about your border in terms of planting groups or drifts. For your front edge, consider alternating groups of 3 Erigeron with groups of 2-3 Lamb's Ear, spacing the plants within each group about 30cm apart. This gives you roughly 15-18 plants for your front row across the 540cm length.

https://youtu.be/h7DfM3xqfR4

Behind this, your Salvias and Lavender can be spaced about 40-45cm apart, as both will develop into substantial clumps. You're probably looking at 8-10 plants for this middle section. The Alliums can be planted in between these larger perennials in groups of 5-7 bulbs, as they'll emerge, flower, and then fade back whilst the other plants fill the space.

Try to add seasonality to the flower bed

One thing I always emphasise in my herbaceous planting guides is thinking about how your border will perform across the seasons. Erigeron Karvinskianus is brilliant because it flowers from late spring right through to the first frosts, giving you that cottage garden feel throughout the growing season. The Lamb's Ear provides wonderful silver foliage structure year-round, which will be particularly valuable during the winter months when your other plants have died back.

Your Salvias will provide that crucial summer colour and structure, whilst the Lavender offers both fragrance and that Mediterranean feel that works so well with the Erigeron. The Alliums will give you early season height and drama, then fade away to let the other plants take centre stage. It's actually a very thoughtful combination that shows you understand how different plants can work together across the seasons.

Managing Expectations and Growth

One crucial point about Erigeron Karvinskianus it's what I call a "vigorous self-seeder." Whilst this can be wonderful for creating that natural, cottage garden look, it can also mean the plant pops up in unexpected places throughout your garden. Some gardeners love this spontaneous quality, whilst others find it a bit too enthusiastic. You can manage this by deadheading after the first flush of flowers if you want to control its spread.

Similarly, your Lamb's Ear will spread steadily via surface runners. In a 45cm deep border, you might find it trying to escape into your lawn or pathway, so be prepared for some annual edge maintenance. This is another reason why a deeper border would give you much more flexibility - the plants would have room to spread naturally without immediately outgrowing their allocated space.

Start with the numbers I've suggested, but don't hesitate to adjust as you observe how the plants develop. Gardening is wonderfully forgiving, and most of these plants can be moved or divided if you need to make changes later on. The key is getting started and then observing how your particular combination works in your specific conditions.

All the best

Lee Garden Ninja