Growing Delphinium in Pots or Containers

Oh Yes You Can!

Delphiniums have not traditionally been grown as container plants in New Zealand but this is a practice often used by delphinium enthusiasts in England and the U. S. A. The method, developed as a space saver and a means by which plants in flower may be transported to shows, or to new garden sites, is also well suited to modern city living.

By growing delphiniums in containers, modern patio gardens can be graced with these magnificent flowering spires which become stunning focal points and fabulous conversation pieces. Growing practices used for delphiniums in pots vary considerably, all with apparent success. In this article I have taken these practices and blended them with our own experience gained in growing thousands of potted delphiniums, to produce a recipe for successful patio, or potted garden delphiniums.

Best pot size:- An 8 litre pot or planter bag is ideal. You can use up to 20 litre if you wish but these are more difficult to move. Some people use the traditional method of growing in smaller pots and potting on as required until the 8 litre size is reached.

Type of pot:- Whatever you like but remember that terra-cotta should be glazed to avoid excessive moisture loss. Pots which have been previously used should be well scrubbed and disinfected with a product such as Jeyes solution.

Plant material:- Always start with the best you can. Solid first year hybrid seedlings from our New Zealand bred New Millennium strain, with good basal shoots are well suited. Cutting grown selected New Millennium hybrids, with strong, relatively short stems are, quite simply, the best there is anywhere and have been extensively tested in pots. We can recommend the best varieties - and the right varieties really are important for the best results. If these plants are potted up in spring they will flower magnificently in the early summer, after which there will be time left to cut away the old stem to allow a second flush before it’s too cold for lingering on the patio with a gin, or whatever.

Potting up:- Follow normal practices for potting any plant, allowing good drainage and planting to the depth of the surface of the potting mix already around the plant. Leave a 2 or 3 cm gap between the soil surface and the top of the pot. This is useful if you water onto the top of the pot. At this stage you may like to insert three stakes making a triangle around the edge of the pot and taking care not to damage the root ball. I don’t do that at this stage but do this later, just before they actually need supporting, if at all, because I don’t like stakes. Does anyone? Sprinkle a few slug pellets on the plant. Yes, on the plant!

Growing Method 1:- We pot our delphiniums into 8 litre bags in the late winter or early spring. As these plants begin to grow and the weather warms up and their roots reach the bottom of the bag, we place the plant (still inside the 8 litre planter bag) in a larger ornamental container. The space between the bag containing the delphinium and the outer pot should be not less than 1 cm wide but otherwise as wide as you like. This space is then filled with pumice or bark. The outer container must have good drain holes. We then place the ornamental container on a large, 3 or 4 cm deep saucer. Watering then consists of filling the saucer and waiting until it is almost empty before watering again. This avoids letting the delphinium dry out, which can be fatal for a large plant, and also has the advantage of keeping the roots cool, as the plastic planter bag will be out of direct sunlight and be insulated by the bark or pumice. On a hot, breezy summer’s day a plant in full flower may need watering morning and night but it’s worth the effort. We recommend using a modern irrigation system for watering all your patio plants, baskets etc. The small investment is well worth it in both results and peace of mind and saves work too. I love saving work.

Growing Method 2:- This is easier but takes more space. A lady in British Columbia, Canada grows her beautiful patio delphiniums from our seed in wooden half-barrels about 60 cm deep and 75 cm wide. She simply fills the barrel with soil and compost and plants three plants to the barrel. She waters every few days, or when they look just a tad dry (Canadians use tads too). The results are amazing - short, stocky spikes with masses flowers on each one, and one happy lady.

Cultivation:- Your potting mix should be the type you would use for potting roses, but after planting I recommend a top dressing of 3 month, high nitrogen, slow release fertiliser (e. g. osmocote), which should be well watered in. Delphiniums are very hungry. Thereafter, once the foliage covers the planter bag, and until flower buds can be seen on the flower spike, a fortnightly watering of liquid fertiliser, say 12:10:10, should be applied via the saucer. Once the delphinium has flowered, cut away the old spikes and the spent foliage and discontinue watering in the saucer. Water only enough to keep the plant from drying out until new growth the size of a fist is produced, thereafter resume watering from the saucer. A tablespoon of dolomite lime could be added at this stage if you’re really keen.

Staking:- Much has been written about the staking of delphiniums and many types of stakes are available. The key points are to ensure that the plants have plenty of support and that all stakes are placed well away from the crown of the plant, and are as unobtrusive as possible. Yeah right! We use green plastic coated bamboo when necessary.

Winter care:- After the second flush of flowers is past (say May/June in New Zealand), cut back the old flowering stems to within 5 cm of the base of the plant, remove the plant (still in the planter bag) from the outer pot and place it in a cool area, but where it will receive some sun. Water only enough to keep the mix barely damp and keep the slug pellets going. Your plant should then lie fairly dormant until early spring and, as new growth appears, side dress with three month, high nitrogen fertiliser, increase watering and place the planter bag back in the ornamental pot. During those winter months it is important to keep the plant as cool as possible as winter chilling is beneficial to delphiniums. Add another teaspoon of lime, even if you’re not keen.