Water for Dahlias

Have you seek the hype; have you caught the craze? Do or don’t you water your dahlias? Water is essential for life and all living things will need water. Each organism on Earth has adapted to adjust for water availability in it’s climate. If you are unfamiliar with where Dahlias come from, then you may want to read our page on their native land. As a Central American plant, they have certainly caught the attention of gardeners world wide. They also seem to be somewhat forgiving and grow in a number of climates, but they may not always survive the winter with out care.

Because dahlias can be grown just about anywhere, that means how to water your plants is not cut and dry (pun intended). Water needs also vary at different stages of the plant’s life cycle. So the advice for one climate may be all together wrong for another climate. We are going to break down watering into three categories for ground planted tubers. You will have to find the climate that is closest to yours and modify as needed. At the end is a note on growing in pots, which is completely different than growing in the ground.

Cool Coastal Climates

This is the world we live in. Summer temperatures yield highs in the 70’s and sometimes 80’s, but the ocean storms keep us wet and socked in for much of the year. Wild Horse Gardens routinely gets 100 inches of rain a year and days in which 5-8 inches of rain falls. This is the total rainfall in a year for some drier climates, so its hard to fathom that dahlias can survive in this drizzly onslaught.

Like anywhere else, we don’t plant until the soil temperature is roughly 60 degrees. Often we struggle to get plants in even when it’s warm enough because the soil is too mucky to till and its too saturated with water. It’s really important in this climate that the soil is warm enough to plant the tuber. With the water inundation the tubers need to quickly grow feeder roots and lenticels (pictured below) to deal with the excess water without rot.

Many in our area over winter their plants in the ground with out rot, showing the power of lenticels in managing the water. You may guess that in this climate, we don’t water the tubers until well after the rain has stopped and the ground water has gone down.

We use a spade and dig a small hole every couple of days about 5 – 6 inches deep once the rains have stopped. If the soil is dark and moist, we do not water. Once the water level has receded below this shallow pit, we start watering.

You can use overhead or soaker hoses. Many prefer soaker hoses to reduce chances of developing powdery mildew. Having good air circulation is critical. At this point in the season, the watering advice to water deeply (down to those feeder roots that are 6-8 inches or deeper in the soil). This means a minimum of 45-60 minutes. In our mild summers, we can get away with this every 2-4 days, depending on the day time temperatures. Warmer days in the 80’s necessitates watering every other day. If we have a heat wave and go to 90 or 100 degrees, then we water daily. Deep watering is the key.

Temperate Arid Climates

All tubers should be planted once the soil is 60 degrees or higher. In a temperate arid climate, where spring time temperatures are 70-80’s, but there are infrequent rains, you do not need to water in the tuber, or do much with it until the sprout breaks the surface. The tuber has stored water to get itself going. If you water its daily, and especially if the soil doesn’t drain well, the tuber may rot without its feeder roots and lenticels. It’s best to plant and forget, but please don’t panic if it rains, or a well meaning spouse tries to do something nice for you and waters your garden. If tubers can survive the water of the coastal environment in spring, then an occasional bit of water will not rot your tubers.

Once your plants are growing, they are going to require more water than the tuber could provide. Here again comes the call for deep watering every couple of days. It takes a good 45-60 minutes to get the water down deep enough to the plants. If you turn off the water too soon, surface evaporation will cause some of the water to be lost.

If you get into hot spell, the plants will need more frequent watering. Dahlias are not accustomed to 90 degree or 100 degree heats. So daily watering, typically in the evening when the plants are less stressed and can recover is a great plan.

Living in this environment, you may never see a lenticel in the fall. If there is not an excess of water, the tubers do not produce this adaptive feature. If you do lift tubers in the fall and see these, consider two things. 1) Have I lifted my plants near to a rainy period in which the tubers would be creating this adaptive trait of 2) is this area in my garden over watered? Depending on your answer, you may adjust where you plant or how you water the following year. I am a big fan of digging a spade and looking at the soil a few inches down. A techy person might get a moister meter probe to do the same thing.

Blasted Hot Arid Climates

It broke my heart seeing people have no tuber growth and desiccated tubers that never grew. Many of the people who reported this issue on the Facebook page Dahlia Growers suffered just this fate in the spring of 2022. They did not water their tubers, but their climate was so hot that the soil dried out, leached the moisture from the tuber and the tuber died.

Growing in hot climates takes a bit more work, but you can be successful with more water. When starting tubers, your spring temperatures will likely be routinely in the 80s and possible 90’s as you prepare for summer heat over the 100’s. You will want to water your tubers at least once a week in the spring. You want the soil to stay loose and hydrated, but not saturated. A moisture probe may be a handy tool to let you see what is happening in the subterrain.

You will likely have to do a deep water at a minimum of 60 minutes every two days. On scorcher days, water deep daily and if there is really intense heat, a morning shower and deep evening water will help quite a bit. If you have days well over 105 degrees, you will want your dahlias planted in a place that gets a spot of shade over the day to give the plants a reprieve. If this isn’t possible, a mid afternoon, 30 minute over head spray can create a bit of a microclimate to help with the stress of the heat.

Growing in a very hot climate is much more challenging because dahlias are divas and they get stressed easily in the heat. Water will be your saving grace.

Watering in Pots

Growing any plant in a pot is different from the ground. Planting in the ground, the subterrain keeps the roots cool and ground water can percolate up through layers providing a bit of moisture. A pot gets hot, which can cook roots and has no reservoir of moisture during the day. If you are planning to grow in a pot, then you need to be in the watering game to make your plants successful. Dahlias in pots can be leggy, stunted, and slow to bloom if their needs are not met. Do this right and you will have the most amazing plants. Again, dahlias can be total divas.

In the spring when you plant your tuber in the pot, make sure that the pot is at least 4 times as wide as your tuber. The root ball of this plant can get quite large. Use a soil medium that has a hydration medium of some sort. This is a substance like vermiculite that helps the soil retain moisture. Depending on your climate, the soil may stay moist on it’s own, or require a bit of watering. In a large pot as described, I add 1 cup of water when I stick my finger all the way into the soil and I can’t feel any moisture. If your soil is so dry you can’t stick your finger in, then its time for a touch of water. This will depend on where you live and what your climate is like. Did your pot get full sun on a hot patio, or was it an over cast day? You can not simply plant and forget in a pot.

If you are starting tubers inside in gallon pots or smaller pots with the intent of planting into the ground, watch the soil using the same finger test described above. For a small 4 or 6 inch peat pot, I put a tablespoon of water in the pot when my finger feels the soil drying out. A gallon size pot may need 1/8 to 1/4 cup of water. You want a hint of moisture to prevent the tuber from drying, but not soggy so the tuber rots.

Once the sprout is up, you will likely need to start by watering a small bit more frequently (like the same 1 cup for the big pot, but more often). This will be enough until your plant has several leaf nodes. If there is a hot day coming up, then you need to water a little heavier. As the plant gets larger and the days get warmer in the summer, you will have to water every other day at a minimum, but likely daily. On a really hot day (anything over 80 degrees on a cement patio getting full sun) will need a water morning and evening. This added moisture in the soil will help keep the roots cool and moist. Water from the soil will evaporate off some heat, so you are watering not just for the plant to drink, but to maintain the climate inside the pot. Your goal is damp soil 5 inches deep, not soggy sloppy soil. Make sure your pot drains well. If you have a scorcher day, consider using something to shade the pot to protect your roots.

If you have any questions, or would like advice – consider emailing us at [email protected]. Please include some pictures of the soil and plants so we can use that data to frame our comments.