Producing Drought-Resistant Dahlias In Shallow Soil

Although dahlias are one of the easiest flowering plants to grow, because they develop very quickly they need a good supply of water. So most growers water their plants very often. But if the roots can access damp soil, this watering will be greatly reduced. I am going to show you how I achieve this.

As you will have seen from previous dahlia posts, two things will help here:

• planting tubers deeper in soil

• helping plants develop a larger root system (not dealt with here; see last year's posts on dahlias, watering and water conservation)
  

One or the other is a real benefit. But we want to have both (if you haven't already done so, please read: 'How To Produce Drought-Resistant Dahlias', posted on 24th March 2011 and 'Dahlias Need Stronger Light', posted last week, 14th April 2011).

PLANTING TUBERS DEEPER IN SOIL
With normal fertile (dark in colour) soil, this is no problem - just dig a deeper hole and bury. Simple. However, things are rarely so straightforward. Lots of growers have to contend with shallow soil. It might not be physically possible to dig a deep hole; or there might be a shallow depth of FERTILE soil (i.e. soil is deep but soon changes to a lighter colour of infertile subsoil) What then? And don't even think about a pick and shovel! Although, I don't know, you could dig a hole and fill it with potting compost. It has been known. But these would be a desperate measures.

Obviously, all we can do is try to get the tubers as low as possible. But this can be a problem, especially with those large clumps of tubers - you know, the ones in soil we awakened.

These clumps are enormous. As you can see, even when stood in a 30cm (12 inch) hole, the shoots are only 10 cm (4 inches) below soil level. Before we go any further, we should be very clear about something:

IT'S THE SHOOT BUD DEPTH THAT MATTERS, NOT THE TUBER DEPTH

Whilst 10 cm (4 inches) is the recommended depth for dahlia tubers, unfortunately this is TOO SHALLOW for our purposes. If we want to reduce watering, we need to somehow get the shoots lower - preferably at least 20 cm (8 inches) below ground.

Sorry. It's not a very clear photograph. But the large clump on the right is a clump of awakened, unsplit tubers - as they were stored over winter. The tray on the left contains the tuber we put there recently. In order to level the shoot buds in both cases, I had to stand the tray on a house brick. The red line represents the level of the base of the the tray. Everything below the red line needs reducing, all 40 cm (8 inches) of it. How?

Clearly, separating the clumps and planting the tubers flat in trays, as you have seen, is good. And we could still separate the clumps into single tubers. Why not do this? Well, it's getting too late*. The clumps will have quite a bit of root growth in their soil, and if we removed this, it would waste time re-growing. Best keep disturbance to a minimum.


Divide clump by slicing down between buds (try to get at least one strong bud on each piece) with a sharp knife or spade. This photo shows a slice laid on its side, cut-end downwards. It can be planted like this. Don't worry about the buds pointing sideways, not upwards: they'll soon turn upwards; and the roots downwards.

Yes. O.K. But this is a smaller clump. We've lost some buds. There won't be as many flowers, will there?

No. But there is a way: Make a dahlia tuber wheel.


DAHLIA TUBER WHEEL

Once the tuber clump has been carefully sliced into thinner sections, it can be re-constructed in a different dimension. Lay the slices, bud ends inward, out like the spokes of a wheel in the bottom of the hole (this one is on the surface). This way, no buds are lost from the original, yet we have achieved our purpose: THE BUDS ARE NOW AT ABOUT 17.5 cm (7 inches) deeper in the soil. How about that, Ben Murphy?

FEEDING
Last year, I laid the tubers on muliti-purpose compost in the bottom of the hole. This provides nutrition to get a good start, then extra water-holding capacity. It's a good policy. I recommend it. However, I haven't used compost, this year - only a handful of bonemeal, as outlined in other posts (please read them all, if you can - including last year's).

Some growers recommend laying tubers on a bed of garden compost. This, in my opinion, is not a good idea. Chances are, it will be too strong and will burn the delicate young root hairs, especially in dry soil. If you want to use garden compost - or any other type of feed - make sure it is kept at least 5 cm (2 inches) away from the roots - and offset to the side, if possible, so the tap root (which goes straight down) does not have to grow through it. I usually scatter a handful of bonemeal around the edges of the hole. This way, the lateral feeder roots are encouraged to search for it. Roots will always select the right course, given the opportunity.

COVERING
Carefully cover over tuber and young shoots with soil from hole. But only to a depth of about an inch. If you can position the shoots on the sunny side of the hole, the rapidly-emerging shoot (due to warmer soil) will have shorter growth.

WATERING
Water covered tuber well to settle soil for a good start. Then only water again if absolutely dry. This will encourage strong root growth.

* This refers to rooted single tubers. If coming straight from the packet, just dig a hole and pop them in. Note that they are best laid on their side, as this gets the shoots lower.