Delphinium Winter Care: Preparing Delphinium Plants For Winter
By: Mary H. Dyer, Credentialed Garden Writer
Delphinium is a stately plant with tall, spiky blooms that beautify the garden in a big way during the early summer months. Although these hardy perennials are easy to get along with and require a minimum of care, a few simple steps will ensure they survive winter cold unscathed.
Preparing Delphinium Plants for Winter
In preparation for winterizing delphiniums, water the plants regularly as winter approaches and continue until the ground freezes so hard it can no longer absorb moisture. Don’t water with a sprinkler; get in there with a hose and let it trickle until the roots are thoroughly saturated.
It’s important that the ground is damp going into winter so the roots don’t become too dry. The plant will continue to evaporate moisture through the leaves, but the frozen ground won’t accept water to replace the lost moisture.
Cut the plants down to a height of 6 to 8 inches (15 to 20 cm.) after the first killing frost in autumn, or if you prefer, you can save this step until spring. A trimmed plant is easier to mulch, but an intact plant provides winter texture to the garden. The choice is yours.
Either way, remove leaves and other plant debris around the plant to discourage disease and pests, including slugs. Apply at least 2 to 3 inches (5 to 7.6 cm.) of mulch in late fall, when the ground is cold but not frozen. Use organic mulch such as bark, straw, pine needles, dry grass or chopped leaves. Mulch protects delphinium in a couple of ways:
- It prevents damage caused by freezing and thawing that can freeze the crown.
- It conserves soil moisture.
Avoid using whole leaves as mulch; they will form soggy mats that can smother your delphiniums. If you have leaves you’d like to use as mulch, chop the leaves up by running a mower over them a couple of times first.
Delphinium Winter Care
Once you’ve watered and mulched in autumn, delphinium care in winter is minimal. It’s a good idea to water occasionally during winter months if the ground thaws enough to soak up water.
If you’re an adventurous gardener, you may want to try sowing delphinium seeds in winter. With any luck, the seeds will germinate about the time winter loosens its hold for spring planting.
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If You Know Secret, Delphiniums Not Hard to Grow
Delphiniums are true garden aristocrats--strong vertical shapes that tower above everything else, in colors that are most regal, including that most precious of garden colors, true blue. No wonder some of the favorite strains have names like King Arthur, Guinevere and Galahad.
They would probably be in every garden if it were not that they have a reputation for being difficult to grow, but they are not, if you know but a few delphinium secrets.
Did you know that they are not light sensitive? That means you can plant them at any time, though right now is one of the most favored seasons, because planted now, they will bloom in late April or May, in harmony with so many other flowers that also flower in late spring. You can, however, plant delphiniums in the middle of summer or the dead of winter. The best flowers come on plants that do their early growing when it is cool, but flowers will bloom in any weather, they will just be on shorter spikes.
Did you know that there are full-sized delphiniums and compacts? Though there are many kinds, two types are commonly available--the Pacific Giants (more properly the Pacific strain) and the Blue Fountains strain. Pacific Giants can live up to their name by growing to five or six feet or even eight on occasion. Spikes are fat and full, or should be.
The Blue Fountains (sometimes labeled Dwarf Fountains) strain grows much shorter, from three to four feet tall, sometimes even less. These too should have fat, full spikes, but don’t always.
The reason plants are not always as they should be is that they are grown from seed that is open-pollinated in fields and they do not always come true to type. Many years ago, the Pacific Giants were as uniform as corn in a field, but then they were hand-pollinated and carefully selected for their seed. Ken Dorwin, now with Hi Mark Nursery in Carpinteria, used to work with Frank Reinelt, the originator of the Pacific strain and he knows how much they have deteriorated over the years. He notes that flowers are not always double so the spikes look less full that the height is no longer uniform and that often they aren’t even the color they are supposed to be. But even then, there are few flowers that make such a spectacle in the garden.
It is important to note that in California we only grow delphiniums from seed-started plants--seedlings--not from cuttings. On the East Coast and in England, most delphiniums are grown from cuttings, but despite the fact that delphiniums are perennials, and that some are native to California, garden types do not grow as perennials here, but must be replanted every year--from plants begun from seed.
This is important to know because you shouldn’t waste time trying to keep the plants alive for more than a year, and you must expect some variability among the plants you purchase. You certainly do not want to throw money away on cutting grown-plants purchased from mail order growers back east.
This variability may change for the better in the near future because Dorwin is busy at work redoing the Pacific strain, so in a few years, it may once again be as flawless as it was 40 years ago. He is also working with Delphinium zalil , a yellow species, and D. cardinale , a red, in an effort to get a delphinium strain that includes red and yellow (the Swiss have already produced some small red and yellow strains but they are not commercially available).
The Pacific strain is actually a group name for several individual strains and each group is supposed to be the same color. For instance, plants labeled as Galahad are all supposed to by white, Black Knight should all have dark purple flower, Summer Skies should have blue flowers with white “bees” (centers), and Blue Bird should be pure blue. But, since these are strains, they will not be indentical, just similar. You may find them simply identified as a mix, which means they could be any color and probably will be.
Did you know that delphiniums like lots of water? More than most flowers in fact, but here we must pass along a trick developed by nurserywoman Pamela Ingram of Sassafras Nursery in Topanga.
She has been adding those new gels to the soil that goes back into the planting hole. These gels, or soil polymers (the most commonly available is Broadleaf P4), can increase the water holding capacity of a soil by much as 400 percent, thus making much more water available to the plants in contact with it. In other words, you can give the delphiniums more water without watering them more often so other plants in the vicinity are not drowned.
Looking a lot like coarse salt, the polymers swell up to the size of peas as they soak up water and store it for the plant’s use. Naturally, you want to put the polymers where they do the most good, near the bottom of the hole, and when you pull the plant out in winter, you will find that the roots have latched on to all of the polymers in their search for water.
You can probably guess just from their size that delphiniums need lots of fertilizer, and they do--another secret of success. The easy way to provide it is with a slow-release fertilizer, Osmocote being the best known.
As the package points out, this kind of fertilizer slowly releases the nutrients to the plants for as long as 120 days--"120-day continuous feeding"--so the delphiniums get their daily bread.
Be prepared for a little expense here--Osmocote costs around $8.50 for 40 ozs. and Broadleaf P4 goes for about $18 a pound--but then only the best will do for these Cadillacs of flowers (the plants themselves are going to look like a bargain at around $1.50 per 4-inch pot).
Here’s how you might plant your delphiniums this year, using this new knowledge and technology: Select a sunny spot with the best soil in the garden, or thoroughly prepare a garden bed by digging in organic amendments until a rich soil results.
The Pacific Giant plants should be spaced about a foot apart, or a little more Blue Fountains 10-12 inches apart. Dig a hole about a foot deep and a foot across for the Pacific Giants 10-by-10 inches will do for Blue Fountains. Ken Dorwin tells us that delphiniums have fairly shallow roots, but the depth of the hole is important to assure that excess water drains away.
Working in moist (but not wet) soil, do one hole at a time and put the soil you dig out of the hole in a big plastic bucket or basin. Add several handfuls of an organic amendment, such as Kellogg’s Gromulch to the soil in the bucket and mix it in thoroughly, breaking up any clods as you go.
Now partially refill the hole with this amended soil until it is almost full enough to support the size plant you are working with. At this time of the year, plants in 4-inch pots are the best bet plants from packs are better earlier in the year and plants in gallon cans are only for those who are in a real hurry (you will be disappointed by size of their spikes). Try to find plants that have not yet made spikes.
With your hands, pack down this soil in the bottom of the hole so it doesn’t settle too much later on. Dump out about half the soil that is left in the bucket and add a scoop of Osmocote (scoops are included in the package) and a teaspoon of Broadleaf P4 (it doesn’t take much) to what remains in the bucket.
Mix this all together, put some in the hole, set the plant on top and then fill in around the plant. If it doesn’t quite fill the hole, use the soil you set aside. Pack the soil sown with your hands. When all the plants are in, water thoroughly and make sure plants don’t dry out even for a moment during the first few weeks until the roots find those polymers.
This may sound complicated but it isn’t (I just did it) and work progresses quickly it is also the kind of garden work one can savor.
There is one more thing to do if you are growing the Pacific Giants, what photographer and delphinium fanatic George de Gennaro calls “rule number 1"--immediately after planting, put in sturdy stakes to support the tall flowers.
They are going to need them as soon as they begin to shoot up (tie them to the stake as they grow) and you had best be prepared (and you do not want to damage roots by shoving in a stake later on).
When they finish blooming, cut the spikes back, leaving only a few leaves at the base, fertilize again (with a granular fertilizer scattered over the bed), and the delphiniums will make new sprouts in a few weeks and begin a second cycle of flowering, though this time the spikes will be smaller though more numerous.
What do you plant delphiniums with? I always think of delphiniums as the church steeples in a village of other flowers, and it doesn’t take many to make a statement. I plant them in small groups of five.
Try them with lower bushy flowers, such as Shasta daisies (which can be planted now and will bloom with the delphiniums) or coreopsis or just about any other perennial. They are equally elegant behind summer annuals such as gloriosa daisies, zinnias and marigolds though they will probably flower before these reach their peak. De Gennaro plants them behind lilies. Whatever you choose, you will not be disappointed, and you will probably never consider planting flowers again without the royal company of the regal delphiniums.
Growing Delphinium In Containers
Even though delphinium plants are more suited to being planted in gardens, it is not impossible to grow them in containers. Perennials usually prefer open spaces because their root systems grow shallow and wide rather than deep. If the plant is adequately cared for and divided as soon as it gets too big, the delphinium can thrive in a container as well.
Step 1 - Prepare the Container
The container must be large enough to permit root development and retain moisture. Clean the container and cover the drain hole, either with the wire mesh or fine gravel, for drainage. Add a few inches of moist soil to cover the bottom of the container.
Step 2 - Add Potting Mix
Add the potting mix to the container and mold it to the sides to create a space for the plant. Use soil-less potting mix or use 1 part peat, 1 part Perlite, and 1 part organic soil for a homemade mix. The mix must be able to hold water and nutrients, but also provide good drainage.
Step 3 - Remove the Delphinium from its Container
Invert the nursery container and gently ease the delphinium out. Do not use tools to loosen the plant because this may damage the sensitive root ball. Loosen the root ball of any roots entangled around the plant and any matting. Note that the best time to transplant a delphinium is in early spring when it is dormant.
Step 4 - Plant
Place the delphinium in the container with the top of the roots just above the soil level and below the rim of the container. Fill the container with the remaining potting mix and gently press down to stabilize the plant.
Step 5 - Stabilize
Insert three stakes, forming a triangular shape, to provide support for the plant. Delphiniums tend to achieve considerable height and will require the support the stakes provide. Choose bamboo stakes covered in green plastic to make them as unobtrusive as possible.
Step 6 - Water and Fertilize
Water the plant thoroughly and apply liquid fertilizer. A 12:10:10 mix is ideal. Delphiniums are very hungry plants, so they must be fed approximately every 2 weeks. If the weather is hot and dry, they will require daily watering, but don't go overboard or the roots may rot.
Step 7 - Position
The delphinium will need access to direct sunlight for at least four to five hours daily, so position the plant near a window that allows in plenty of light. If you plan to place them on a porch or patio outdoors, the delphiniums will have to winter indoors because the container does not provide sufficient protection for the roots against the cold weather, which will inevitably lead to root damage. The container can also be insulated against the cold weather with bubble wrap if the plant cannot be moved indoors.
Delphiniums are beautiful annuals, and with proper care, can last up to five years in their containers. As with garden planting, the delphiniums will need to be divided every three years, maybe even sooner depending on how quickly they grow. You could also transplant them into larger containers.
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Digging and Storing Dahlias: How To Dig Tubers Up
Cut foliage back to a couple of inches from the ground, before digging your tubers up.
- Cut foliage back, so that only a couple of inches remain above ground.
- Take your preferred digging shovel and dig around the tubers, being careful not to accidentally sever the roots. Many gardeners use a pitchfork to prevent this from happening.
- Once you’ve dug the tuber up, shake the dirt off and set aside.
- Repeat until you’ve dug all of your tubers up.
Gently shake excess dirt off of your tubers after digging them up
3 Delphinium Pruning Tips
The delphinium plant blooms in early June and requires pruning to promote a second flowering as well as more blooms.
Removing Faded Blooms
Removing faded blooms from a delphinium not only cleans up its appearance but also interrupts the reproductive cycle of the plant. The fading of a flower is the climax of a plant's growth cycle when seeds are released to propagate the plant. If this process is interrupted, the plant begins its cycle again, thus encouraging repeat blooms. Remove spent blooms by pinching flower tips and gently pulling until released. Cut with pruning shears if they do not come away easily.
Cutting old stalks back will give the delphinium renewed vigor and encourage a second flowering later in the summer. The remaining stalks must be removed at the crowns when the plants foliage begins to turn yellow. Ideally, for the delphinium to reach its full potential, only one flower spike should be allowed to develop in the first year, three in the second year and five in the third year. The delphinium is one of the few plants that will flower to “exhaustion” and this sequence will allow the plant attain its best.
To weather the winter safely and to ensure a full bloom for the following year, the delphinium must be cut back hard. Stems should be pruned to within 1 or 2 inches of the ground.