Caring for delphiniums

At this time of year, delphiniums stand about in the garden like rockets or fireworks that have left their blazing trail standing in the air – the most vigorous and meaty verticals the summer garden can provide. Eremurus and foxgloves are somehow gauzier and more delicate, making less of a solid block, but that big clumpy, forest-like density of delphinium colour is their great contribution.

Julian and Isabel Bannerman – the great garden designers – have made a bed devoted almost solely to delphiniums in their own garden at Hanham Court. In their characteristically confident style their bed is not 5ft long, not 20ft, but a 50ft expansion of every tone of blue.

They were inspired by the great show stands of specialist nursery Blackmore & Langdon, one of the highlights for years at Chelsea Flower Show (RHS exhibitor since 1903), but the Bannermans' church-backed bed in their flower-crammed garden surpasses even that. Each plant – now three years old – stands well above my head, and they are in colour blocks, three or four plants in each group.

Many of us are tentative about using delphiniums together as their show is magnificent but brief, but – if we have the room – we should rethink that. This month of concerto is worth the other 11 months of quiet. If you know the plants you like, back them all the way, fill whole areas of your garden with just them and revel in it. That's the Bannerman way and I am now rethinking my garden at Perch Hill with this in mind.

How to get a great display

To have delphiniums looking as good as this requires a bit – but not too much – TLC. They love well-drained soil, but are hungry feeders. This means adding plenty of grit on heavy soils to help with soil drainage. If delphiniums sit wet in their dormant winter season, they can die, so make sure you prevent this.

They also need plenty of organic matter added to their planting holes when they first go in, then regular feeding throughout their growing season. You want to use something slow release – blood fish and bone, or a scattering of bone meal. Do this a couple of times in summer and try to repeat once in autumn before the plants die down.

Pests and diseases

Delphinium shoots are slugs' spring asparagus. The plants break through the ground in March and if you don't watch out, slugs can have every one. That's the moment for your nemaslug, or a dose of garlic.

Experts at Blackmore & Langdon say they've recently heard about an effective organic slug treatment – garlic drench. Make your own with two crushed garlic cloves added to two pints of water. Boil for half an hour and then strain off the liquid and bottle. Plastic milk cartons are good. Keep the liquid in a cool place and once a week, in their early growing season of March and April, dose your plants from a watering can. Add two teaspoons of drench to one gallon of water and water the plants and a good circle of soil around them.

In a dry year, the delphiniums in my garden suffer from powdery mildew. I love the deep rich blues and purples such as 'Chelsea Star' but these are the most prone to this fungus. For prevention, plant your plants widely and thin out the shoots emerging from the crowns early in the year to allow for air circulation. Nick Langdon, one of the Langdon family still working in the nursery, says the only sure prevention is a monthly dose of a systemic fungicide.

He also reports he has heard good things about an organic treatment for mildew based on washing powder. Mix up one cup of bicarbonate of soda with one cup of automatic washing powder in a bucket of water. Then mix a cup of that in one gallon of water and use to douse your plants once every couple of weeks to keep mildew at bay. The washing powder helps the mix stick and the bicarbonate burns the mildew.

Staking and support

The other vital thing with delphiniums is the staking. Any strong wind or heavy rain will flatten them and break their hollow stems. Nick Langdon recommends creating a cage from three bamboo canes, trussed up with string. This looks fine in a full border where you don't see the base of the plants, but in the Bannermans' garden, the staking is a work of art.

Julian doesn't like the shiny texture and colour of the canes against the bright green of the leaves. He paints each of his 8ft canes with an undercoat of brown, then a second coat of a mellow sage green. Then while the plants are growing fast in May, he lowers round metal noughts and crosses grids to a height of about 4ft to 5ft over the canes and attaches them with cable-ties. The delphiniums grow through the grids, which become almost invisible.  You can also buy flower rings which work on the same principle.

As cut flowers

There's one final fact that might be useful if you're inspired to create a delphinium collection. For a wonderful June treat, fill at least one vase. If you are brave enough to pick them, you want the flowers to last as long as possible.

Several years ago I went to the delphinium society competition at RHS Wisley. There all the competitors ''stage'' their stems, sometimes working all night to get them perfect. Their collections are then judged before the show opens to the public and they need to stay looking good for at least another week. How do they guarantee this? All the hollow stems are filled with water. They put their thumb over the end of each one and lower them into a vase full of water. The stems last well, with the individual flowers even at the tip of the spike lasting at least a week.

Shop now