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 days--"day continuous feeding"--so the delphiniums get their daily bread. The Pacific Giant plants should be spaced about a foot apart, or a little more; Blue Fountains inches apart. Dig a hole about a foot deep and a foot across for the Pacific Giants; by 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. 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. Mix this all together, put some in the hole, set the plant on top and then fill in around the plant.
Pack the soil sown with your hands. 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 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. Once a week add two teaspoons of drench to one gallon of water and water the plants, drenching a good circle of soil around them.
In a dry year, delphiniums can suffer from powdery mildew. For prevention, plant your plants widely and thin out the shoots emerging from the crowns early in the year to allow for air circulation.
You can also remove any affected leaves and try an organic solution such as homemade comfrey tonic to keep mildew at bay. Delphinium black blotch is a bacterial disease that can be a problem in wet, humid summers. If a delphinium is affected, lift it and dispose of it, and avoid replanting delphiniums in the same spot. Slugs love delphinium shoots so make sure you protect the plants early in the season.
Delphiniums that have sat cold and wet through winter may die, so ensure good drainage and use a mulch over winter. A slug attack when the plant is young might also impact vigour and cause the plant to fail. You should also check for black blotch. Ensure the plants are well watered during droughts. Also make sure they are staked effectively — the hollow stems combine with heavy flowers can mean they can bend and even snap.
Yes, delphiniums are very toxic to dogs, other pets and humans. Flowering time for delphiniums is June and July, with a second flush of flowers in August and September if cut back properly.
Delphiniums do self seed but are unlikely to come true to the parent plant. Yes, given proper care, delphiniums will return year after year. Staking and support is essential for delphiniums as they are tall, heavy plants with hollow stems. Alternatively, you can buy a plant support ring or a plant support grid — placed at about 1m 3ft high, the grid allows the delphiniums to grow through and up, and becomes almost invisible as they grow.
The easiest way is to harvest fresh delphinium stems and then hang them upside down somewhere cool, dry and dark such as a shed or cellar. The petals should dry in a couple of weeks and can be gently pulled from the stems.
Delphiniums make great cut flowers in summer and will last up to 7 days in a vase. Pick when most flowers are open. For tall spires, insert a cane into the hollow stem to avoid it bending and breaking. Alternatively, use water — sink the stem into the vase, then put your thumb over the end of each stem and lift it out. Then lower back into a vase full of water. Delphiniums struggle in winter wet, so add grit to heavy soils to aid drainage. Plant your delphinium at the same depth it was growing in the pot, and back-fill with soil, firming in gently.
Water in well. Delphiniums are hungry plants, which means that they need feeding regularly. Apply a liquid fertiliser every couple of weeks once the first shoots appear to encourage strong growth. Spring is also the best time to stake taller varieties, either with canes or using a frame that plants can grow up through.
Although delphiniums have a relatively short flowering season, cutting the flower spikes back as soon as they have faded can encourage plants to produce a few flowers in late August or September. You can grow delphiniums from seed. Many varieties are available, or you can try saving your own. Sow from February to June, or in September or October. The best method of propagating delphiniums is to take cuttings from the base of the plant basal cuttings in spring. In spring, as soon as the new shoots appear, protect them from slugs and snails — they will demolish the fresh green new growth of young delphiniums.
Use a garlic drench, organic slug pellets, or a biological control which is only effective against slugs. Delphiniums can be susceptible to powdery mildew in dry weather, so keep the soil around them moist. They can also be affected by rust, which causes rust-coloured pustules on the foliage.
Pick off any affected leaves as you see them and clear up any fallen leaves in autumn. Delphinium black blotch is a bacterial infection that causes large black blotches on leaves.
It is particularly common in wet summers and can affect the health of the whole plant. The only treatment is to remove affected plants to prevent the spread of this disease.
It makes a spectacular cut flower. They are shortlived perennials. Grown together they make a dramatic display, and they also make fantastic cut flowers. They come in a range of colours, often with a contrasting eye at the centre.
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