Growing Dahlias… in the High Desert of Southwest Colorado
Here are the basics and a few tips/tricks for growing dahlias:
Location + Soil Preparation
Dahlias prefer a location with full sun, some partial shade is OK, and an area with well draining soil.
Prepare your soil - we like to add compost, kelp meal and pelletized chicken manure to our beds before planting. This all gets incorporated and then raked into a flat bed top. If you’re working with new soil, and/or unsure what your soil may need you can do a soil sample and send it in to get tested. We like to use Ward Labs for our soil tests - and you can ask them for advice on how to amend your soil for the crop you plan to grow.
When to plant? This will depend on where you are located. Ideally, you want the soil to warm up and be 55-60 degrees and you want the chance of frost to have passed. Dahlia tubers can rot if they’re planted in soil that is too cool and/or too wet. The tubers will be underground for at least a week or two after planting… so you can push it a little and try planting them 7-10 days before your anticipated last frost, if the weather cooperates. If you have a greenhouse, you can plant roughly a month earlier than in the field. At our farm, our ‘normal’ frost free window is June 1st… so we plan to have our greenhouse dahlias planted the first week of May and our field dahlias planted around 5/21. These dates are moving targets depending on weather and moisture. And, if you see a week of rain and cooler temps coming - it is probably best to wait until after that storm cycle passes before planting.
How to plant? Dahlia tubers should be planted 12-24” apart and 4-6” deep. We plant our dahlia tubers in beds with two rows spaced 18” apart and we plant the tubers 18” apart from each other in row. Once a bed is raked and prepped, I like to take a pointy hoe and mark my planting lines, this helps me maintain a semi straight line as plant. I then place find my labels and dahlias and start laying them out. I then come back and dig a hole and nestle each tuber into the soil - you want to make sure you plant the tubers horizontally with the eye/sprout facing upward. Cover the dahlia tuber with soil and move on to the next one.
Irrigation - Most of the information you will find says to plant your dahlias and to not irrigate them until the sprouts appear above ground. The logic is that if the soil is too wet and/or cool before the dahlia is growing, the tuber could rot. This seems to be true in a lot of areas but can be a tricky balance in the high desert - a lot of the time our soil can be very dry at planting time. If this is the case, you can do two things:
You can pre-irrigate the planting area about a week before planting. This will give the soil a chance to warm back up after you irrigate, or
You can irrigate once after planting and then wait for sprouts to appear above ground before watering again.
I share this info as many years ago I planted tubers in very dry soil and walked away. I waited for sprouts to appear before I even thought about watering them. It was a warm, windy and dry spring and the tubers shriveled up and never grew. So, it’s a balance and varies a lot depending on your location and what the spring is like. The last few springs here have been very dry and windy… but this spring is looking cool and rainy.
Once your plants are up and growing, you can start to irrigate them regularly. This will start out less early in the season and then increase as the plants get larger. Be sure to dig into your soil a few inches down and feel the soil to determine if it needs to be watered. What is happening at the surface can be very different from what’s happening 3-4” down.
Pinching Dahlias - When your dahlias are 12” tall, come through and cut the central growing shoot out, being sure to leave 3-4 pairs of leaves below. This will encourage the plant get stockier and branch out as it grows - and in theory produce more blooms.
Supporting Dahlias - Dahlias are impressively large plants - but they can get top heavy and break. To help your dahlias withstand strong winds it is best to provide them with some additional support - this can be in the form of (1) tomato cages, (2) wooden stakes or (3) if you’re planting quite a few you can setup what is referred to as a corral with t-posts on the edges of your bed and then use twine at 12-18” intervals to keep them supported/contained.
Harvest for Continued Blooms - It is important to harvest your dahlias regularly to help them continue flowering. As my first farm mentor would say ‘picking encourages growth’ and it is so very true. Pick those beauties for your house, your friends, your family, and they should continue to send out more buds up until frost. When harvesting, you can cut deep into the plant to encourage the production of blooms with long, strong stems. And, be sure to go through and once a week and deadhead any flowers that have passed to keep your plants flowering.
Weeding - And, don’t forget to weed your garden or planting beds. I takes us about two good rounds of weeding before the dahlias plants are large enough to shade out the competition, but focus on keeping the planting area free of weeds up until that point.