13 Mistakes to Avoid When Growing Delphinium This Season

Are you growing delphinium this season, but want to make sure you have absolutely beautiful blooms? In this article, certified master gardener Laura Elsner walks through the most common mistakes both novice and experienced gardeners make when it comes to growing delphinium in their garden!

Delphiniums Growing With Garden Mistakes

Delphiniums are the stars of summer. After the peonies finish off spring with a bang, it is the delphinium’s turn to do their thing. These tall spikes of flowers come in soft shades of purple, pink, red, white, and even true blue (the rarest color in the garden).

Delphiniums are popular perennials that grow gracefully tall and make garden beds pop with soft color. But they are also prone to a few common gardening mistakes that many novice gardeners and experienced gardeners will make. Making some of the most common mistakes can have unfortunate consequences.

The most important way to avoid problems ourselves as gardeners is to understand what not to do in the first place. In the following article, you’ll learn about the most common mistakes that could be preventing your delphinium from reaching its full potential. Let’s jump in!

Using The Wrong Soil

hands holding soil
Delphinium does not grow well in heavy clay soil and sandy soil.

The wrong soil is the root of many problems in the garden, so let’s start there. Delphinium does not grow well in heavy clay based soil.

The easiest way to test soil is to just get out there and feel it. Take a handful of your garden soil and squeeze it in your hand. If it takes on the shape of a ball after you release it, it has a lot of clay in it. If it crumbles away gently and falls apart, it is great for delphinium. However, if it just sifts away in your fist, it is too sandy.

Clay soil is too heavy and will hold onto water. If delphiniums are sitting in standing water, their leaves will be yellow and they can develop root and crown rot.

Sandy soil cannot hold onto water and your delphinium will dry out too quickly. Sandy soil also doesn’t provide much nutrients for the plant to grow. Some delphinium grows over 7 feet in a single season, this requires lots of nutrients from the soil.

To achieve this perfect soil that crumbles away you will need to amend your soil. If you have the heavy ball of clay, work in either peat or coconut coir in the beds. If you have sandy soil, consider adding loam (black earth). In both instances top dress with a nice rich compost to boost the soil and add nutrients.

Not Providing Enough Water

watering delphinium from sprinklers
Delphinium prefers evenly moist soil.

Delphinium does not like drying out. They will thrive in evenly moist soil. Depending on the area you live, and how much rainfall you receive, will determine how much supplemental watering you will need to provide.

If you live in a dry area, or it has been a dry season, consider mulching your delphinium as a way to help retain the water. Place the mulch at the base of the plant in early spring or fall.

If it is a dry season, provide the water a couple of times a week. They will be droopy if they need water. Aim for the base of the plant when watering as opposed to the leaves. Constantly wet leaves can result in powdery mildew. I like snaking a drip hose through my garden and turning it on for a couple of hours a week.

Watering Too Much or Too Often

water drops on the delphinium blue flowers
With excessive watering, the delphinium can become susceptible to powdery mildew.

On the flip side, too much water can be equally detrimental for your delphinium. Too much water will lead to weak delphinium stalks. The bottom leaves will yellow. They will be susceptible to powdery mildew and crown and stem rot.

First, make sure your soil is not too heavy and full of clay. Make sure the soil is light an fluffy and able to provide adequate drainage.

Do not pick a boggy area of the garden for delphinium (like a low spot, or under a drain spout). There are other plants that can go in wet areas. But delphinium does not like sitting in water.

Improper Watering

watering blue delphinium in the garden
It is recommended to use a soaking hose to water the delphiniums so that water does not get on the foliage.

A big mistake when watering delphiniums is spraying water all over their foliage. Of course, if it rains, not much can be done. But when you are in charge of watering, opt for watering the base of the leaves without spraying the foliage.

Constantly damp foliage invites fungus, disease, and bugs. I use a soaker hose snaked through my garden and I turn it on for a few hours 1-2 times a week, depending on the weather.

Too Much Shade

light blue Delphinium in full bloom
Delphinium needs a lot of full sun otherwise they can be thin and not bloom.

Delphiniums are full sun flowering perennials. Which means they like at least 6 hours of direct sun per day. If they get much less than that they will be very spindly and will not bloom. If you have scrawny delphinium that are not blooming, take a look at how much sunlight they are receiving.

Also, keep in mind that gardens evolve over time. That young sapling you planted 10 years ago has now grown into a big tree that shades the once sunny spot the delphinium is planted in. So over time, sunny areas can become shady.

So if your once beautiful delphiniums are no longer blooming, take a look to see if the light conditions in your garden have evolved. Transplant them to a sunnier area to get them blooming again.

Too Much Hot Sun

blooming Delphinium in full sun
In extremely hot areas the delphinium can get fried, so make sure it gets early morning or late afternoon sun.

While delphiniums are tough full sun plants, the amount and intensity of the light does make a difference. If the sun in your area is too hot and intense,  your delphinium will fry.

If you can place them in an area where they can receive lots of early morning and/or late afternoon sun with dappled shade to protect them from the midday sun, that is ideal. I find that placing them in a garden with lots of other plants and trees is an ideal situation.

Planting them somewhere on their own or out in the middle of a blazing hot garden will kill them. Bleached out and crispy leaves are signs of too much sun.

Not Enough Space

Red and Pink delphinium blooming in garden
Delphinium needs to ensure the flow of air between the plants.

Delphinium is very susceptible to powdery mildew, if you don’t allow for good airflow between your plants you are inviting mildew.

If your delphiniums are becoming thick and overgrown, it is time to divide them. In early spring or late fall, dig up your delphinium and separate them into clumps.

Transplant them throughout your garden or give them to friends and neighbors. This will keep the clumps in check and allow adequate airflow between the stalks. This will lessen the likelihood of developing powdery mildew and/or rot.

Planting in Windy Locations

white Delphinium
Plant the Delphinium in a place protected from the wind.

These plants are not meant to be blown around constantly. The flowers and stems will snap and break. Plant delphinium in sheltered locations. Along fences, or protected by a house is perfect. Or plant them into a protected garden bed.

Planting in The Wrong Garden Space

Blue Delphinium blooming in garden
When choosing a spot in your garden, pay attention to the variety of your delphinium.

While there are dwarf varieties of delphinium, which I will get into in the next section. Some varieties will reach up to 6 feet tall or more. Make sure you plant them in the back of your garden. Also, some varieties do self seed and wander a bit.

If one makes its way to the front of a bed, dig it up and move it back. Delphiniums are great backdrop plants. If they are in the front of the garden you will be missing an opportunity to place other plants in front of it to create layers of foliage and blooms.

 This one may seem obvious, but it is a mistake I see new gardeners make. Make sure to read the tags on plants you purchase and note the height and width of them.

Picking The Wrong Variety

Lilac Delphinium flower with red Delphinium blooming
Delphinium Elatum produces showy flowers that reach up to 6 feet in height.

The beauty of delphinium is there are so many varieties. Make sure you pick a good variety for your space.

Delphinium Elatum are the classic varieties of delphinium (in my opinion), these varieties feature the large stalks of showy flowers that can grow up to 6 feet tall. These are the varieties that look great in the back of garden beds.

There are also smaller dwarf varieties that only grow 3 feet. Some of the alpine varieties are small and nestle into rock gardens. These varieties look great in the front of garden beds.

Ignoring Pests & Diseases

delphinium worm
The most common delphinium pests are aphids, cyclamen mites and worms.

Delphiniums are perennial, meaning they will come back year after year. I often say to just pull annuals that get pests or diseases. But with a beautiful delphinium, I recommend treating it. The best way to do that is to first diagnose it, then treat it, and then find ways to prevent it in the future.

There are a few pests and diseases that can affect your delphinium plants. Some common ones are powdery mildew and aphids. They can also be affected by cyclamen mites, which is very hard to treat unfortunately.

For powdery mildew, prevention is key. Having loose, free draining soil. Watering the bottom of the plant and not the foliage. Keeping the plants in full sun. As well spacing the plants to allow airflow are key to prevention.

If you notice powdery mildew on your plants, use a copper fungicide spray to get rid of it. In the fall cut down the plants and dispose of the leaves and plants. Next season consider the reasons the mildew formed and try to work on preventing it.

For aphids I would start with a good blast with the hose. Then move to spraying with insecticidal soap. Aphids attack weakened plants, so try and figure out why your delphinium isn’t growing in its optimal conditions. Lack of water is a big one. Delphiniums do not like drying out completely, this will leave them vulnerable to attacks from aphids and other insect pests.

There isn’t much treatment for cyclamen mites at this time. Dispose of any affected plant and start again before it spreads. Signs of cyclamen mites on your delphinium include curled leaves, stunted growth, and withered and blackened foliage and flowers.

Since there is not much you can do to treat your delphinium, be diligent when choosing plants at the green house and avoid any plant that displays any symptoms.

Not Staking Your Plants

staking Delphinium
To maintain a tall delphinium, it is necessary to establish support for it in the form of stakes or large hoops.

If you have a large, big blooming variety, they will most likely need staked. The smaller, dwarf and alpine varieties, will do just fine on their own.

Know what variety you have and stake accordingly. You can just get bamboo (or other) sticks and tie the delphiniums to them. Or they make large half moon hoops that can be used to prop up the large plants.

Another option are tall versions of a peony cage. Make sure you put the cages on when they are first emerging from the soil. It is hard wrestling them on to fully grown plants.

One of my favorite staking options is the squiggly poles that a delphinium grows up and can just nestle up against (look for these in garden centers).

Not Deadheading

delphinium seed pods
Snip off the stalk of the seed pods after the delphinium has stopped flowering.

 Deadheading delphinium will give more energy back into the plant itself and not into seed production. After the delphinium blooms, snip off the stalk of seed pods.

This might even give you a second smaller flourish of flowers later in the season. If you live in a region with a long growing season, cut them right back and they will regrow and provide a second batch of flowers.

Some varieties of delphinium will self seed, by snipping them off before they go to seed will keep them from spreading around the garden (unless that is your goal). Note that if you do allow your delphiniums to self seed they will usually not be the same as their parent plants.

Final Thoughts

Delphinium are a garden classic, they are fairly low maintenance. With proper attention, these beautiful flowers can grow both tall and beautiful. If you avoid these common mistakes you should be successful with growing big and lovely delphiniums.

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