Almost every gardener grows spring bulbs like tulips, daffodils, hyacinths and crocus. Millions of new bulbs are sold every year and yet people do not agree on the best time to plant them. Some want to plant early, as soon as they arrive in shops. Others say that you should wait until the ground gets an early frost. Some wait until the ground is fully frozen, but that is usually because they forgot to plant earlier.
A couple of years ago I was in a large nursery that specialized in spring bulbs. It was early September in a zone 5 climate and a manager told me not to plant the bulbs for a couple of months until we had a light frost. I looked him in the eye and asked about the bulbs I had planted in prior years – they were already in the ground. Do they need to be dug up so I could re-plant them after frost? He did not have an answer.
Lets have a look at the science and figure out when you should plant spring bulbs.
The Life Cycle of a Spring Bulb
To determine the best time for planting bulbs it is it is important to understand the life cycle of a bulb.
In winter, the bulb is under ground and not doing very much – it is resting. In spring, it starts to grow and most of our common spring bulbs produce both flowers and leaves. This requires a tremendous amount of food energy which comes from the bulb. The new leaves start to photosynthesize to replenish the food used for early spring growth and to make seeds, another energy intensive process.
By mid-summer the plant has finished its above ground growth and the leaves start to die back. What you do not see is that the roots also die back and stop working. This makes sense since the plant is about to enter into a dormant phase where roots are not required and keeping them is a liability. The plant is now in a state of rest.
Towards the end of summer the bulb senses it is time to start growing again. It first makes roots so that it can have access to water and nutrients. Soon after root initiation, the bulb starts to grow leaves and flowers. These grow until they reach a point just below the surface of the soil. The first time I heard this I didn’t believe it. Why would the bulb grow leaves in fall? So I went out to the garden in November and sure enough almost all my bulbs had green growth just below the surface of the soil.
In hindsight this all makes perfect sense. Most spring bulbs need to start growing early in spring so that they catch the light from the sun before grasses, shrubs, trees and other larger plants shade them. The best way to get an early start is to complete a good part of the growth in fall.
When things get really cold, the bulb goes into a dormant state again, until spring.
When Do Roots Start To Grow?
In late August, zone 5, I happen to dig up a number of bulbs this year and almost all of them had growing roots, including daffodils, snowdrops, Iris reticulata and muscari. Bulbs that are already in the ground don’t wait until late fall to grow roots.
The above picture shows daffodil bulbs that were dug up on October 9, showing both root growth and stem growth.
Garlic is not exactly a spring bulb, but the life cycle is similar. The picture below shows seedling cloves that were not harvested until September and they already have long roots and shoots.
To learn more about growing garlic have a look at Growing Garlic. Also see, Best Time To Plant Garlic.
Dr. Willaim Miller of Cornell University has looked at the affect of temperature on root growth. They took tulips, daffodils and hyacinths; exposed them to different temperatures and then compared root growth. All three types of bulbs displayed similar results. The picture below shows that root growth is very temperature dependent and best growth was seen at the highest temperature tested, 10C (49F).
Imagine what would happen if you plant new bulbs once the ground has frost? There would be little or no root growth.
Required Cold Spell
You are probably familiar with the fact that most spring bulbs need a certain amount of cool weather in order to initiate flowering. In warm climates these bulbs grow leaves but no flowers. This problem can be overcome by keeping the bulbs in the fridge before planting.
A fact that is much less known is that the temperature used for conditioning bulbs is also critical. If the temperature is either too warm or too cold, they won’t flower. This critical temperature is different for different bulbs. Tulips can be condition at lower temperatures than daffodils and hyacinths (ref 3). The latter two condition very slowly below 2C (34F) and if planted in cold soil they may not flower well in spring.
Planting Spring Bulbs Early
I am a big believer in taking advice from nature. If bulbs that are already in the ground start making roots in August it tells me that this is a benefit for the plant. Starting growth early allows the bulb to make a larger root system before the ground gets too cold. Larger root systems must benefit the plant, or they wouldn’t make them.
The exact reason for making roots early may not be known, but it is clear that bulbs prefer to make roots early in my climate and soil conditions. It then follows that new bulbs should also be planted as soon as possible (usually not available until August).
Reasons For Planting Late
If you ask people that believe in late planting for an explanation you will not get much of one. Some people think that the bulbs need to feel the cold before going into the ground. That doesn’t make sense. How much cold can they experience when you walk from the house to the garden and plant? No one suggests leaving them out in the cold to get some frost, which would harm some bulbs.
I think that this idea of giving the bulbs some exposure comes from the fact that tulips and other bulbs will not flower in warm climates. They do need a chill period during winter to flower. Knowing this, it is easy to understand why people might think that it is best to plant after a frost.
Explanations like, “that is how I have always done it”, don’t hold much water either.
Another explanation is that early planting will expose them to too much warm weather during Indian Summer, resulting in early growth and flowering that will be harmed when it gets cold. Consider the bulbs that are in the ground from last year. They experience the heat of summer every year and that does not result in fall blooms. Many of our spring bulbs originate in climates that have hot summers.
I found this statement on line, “They’re dormant when you get them and break dormancy only after the chilling.” This is simply not true as you have seen in the above examples. They break dormancy in late summer before the chill of winter.
None of the above are good reasons for plant spring bulbs in late fall, but there might be a good reason that few gardeners know about.
Bulb Diseases
To better answer the question about best planting time I contacted Dr. William Miller at Cornell University. He has been studding spring bulbs to better understand how they react to various temperature regimes. He is also an expert on tulip diseases.
Most spring bulbs can get a Fusarium fungal infection (ref 2). Each type of bulb has its own species of Fusarium, but they behave similar to one another. The fungus grows best in warmer temperatures. So planting late fall will reduce the incidence of Fusarium, resulting in healthier bulbs in spring.
Where does Fusarium come from? It can live in soil and if the soil is highly infected, it may remain infected for 10-15 years – this is not clearly understood yet. Planting annually in the same spot using the same type of bulb can create a spot where the bulb does not do well because they get killed or damaged by Fusarium.
You can image that this disease is a major problem for bulb producing companies who replant yearly. As a result, Fusarium is frequently found on new bulb purchases. This is one reason why it might be a better idea to plant new bulbs later, once the soil is cooler.
I asked Dr. Miller if Fusarium could explain why some people don’t see any spring growth from their tulips and he said yes. Is it possible that squirrels get blamed for robbing bulbs when the real problem is disease?
If you are planting bulbs and you see any kind of fungal disease on the bulb it is best not to plant them. Discard them in the garbage. Reference 2 shows pictures of the fungus on tulips and gives a good description of the disease.
What about bulbs planted in previous years – can they get Fusarium? I have not seen a clear answer to this. Since the fungus lives in soil it is certainly possible, but it seems as if the major source of the fungus is handling during the production process. Once you have clean bulbs and plant them, it is less of a concern.
When Should You Plant Spring bulbs?
For root growth and bulb development, early is better. For disease control, late is better. The best time is a compromise between the two.
Dr. William Miller made this suggestion, “Soil temperature for planting should be under 15C (59F), and for tulips 13C (55F) would be better. Below 9C (48F), root growth is reduced as temperatures get cooler. Root growth for most spring bulbs is nearly zero at 0-1C (33F).” In zone 5 the best time to plant is October.
If you are digging up your own bulbs, and disease is not a serious concern, they should be planted as soon as possible. There is no advantage keeping them dry until the soil cools down. Any damaged or diseased bulbs should be discarded.
If bulbs are not planted on time, they can be planted until the soil is frozen solid. As Miller says, “late planting is better than not planting.” If you still have bulbs after the ground is frozen, plant them in pots and keep them in a cool spot. They will flower in spring and can be planted out the following year.
References:
- Dig no more: Just till 2 inches for tulip bulbs; http://news.cornell.edu/stories/2011/10/researcher-offers-toil-free-tip-plant-tulips
- Fusarium in Tulips;
- Preparing for The 2015 Tulip Forcing Season; www.flowerbulbs.cornell.edu/newsletter/33-2014-october.pdf
- Photo source – main; public domain
This explains something I have always believed and told people. Literally every single winter, someone says “OMG my daffodils are coming up too early” when in fact, they almost always start emerging at least a little well before spring (typically in my zone 6/7 garden, January). If they get too tall too soon, yeah that can be a problem, but typically no worse than some dessicated leaf tips.
The statement that fall bulbs need a winter dormancy refers to their flowers. Like many temperate zone, flowering trees, the flowers won’t appear unless the bulbs experience a certain number of hours of chilling, which is temperatures between about 30 and 45 degrees F. Plant roots generally do not have a dormancy.
I have dug up both daffodils and colchicum in summer to see what the bulbs are doing and there seems to be little root growth at that time of year. Agreed they might not be completely dormant.
I was advised by my Ukrainian supplier of Dutch produced tulip bulbs to lift them and dry them every year once leaves and flowers have died back. Then replant them in late autumn. He maintains this will ensure the bulbs flower every year. Is this lifting of dormant bulbs really necessary? He maintains the Dutch producers give this information but apparently it does not apply to other Dutch produced bulbs such as daffodils iris crocus and snowdrops..
I have never heard that advice, and doubt it works. The problem with tulips is that many of the modern hybrids are short lived and don’t rebloom very well.
I live in a area where I have a lot of squirrels and chipmunks and have found that the later I plant my tulips and crocuses, the less of a problem I have with animals digging them up. That is the main reason I wait till late fall (or later) to plant bulbs.
Oh no, I didn’t plant yet …. It is October 27th, ground is not frozen in Calgary yet and we are still having mild days. what should i do , plant them or wait till next august? Leave the in the fridge till next Fall?
Thanks
By next August they will be dried up. Late planting is better than no planting.
Valuable stuff as ever, Robert.
I am in the camp of planting any new bulbs on the day of delivery!
Certain bulbs such as snowdrops hate to be brought out of the ground into the dry – and so called ‘bulbs’ such as hardy cyclamen are the same. Both are best planted ‘in the green’ but if not, need to be planted straight away
Certain bulbs such as ipheon are small and might soon shrivel, some anemones become hard in the dry and are reluctant to start up.
Some bulbs such as tulips might be better with late planting because of potential disease problems – especially in very wet conditions. My friend who grows superb tulips says he loses too many to squirrels and rodents and all, if he plants before December.
I think you have written before about the advantages of not lifting bulbs every year. Mine stay in for ever unless I am shifting them!
Agreed some bulbs like snowdrops dry quite quickly – learned the hard way one year.
Excellent article. Perhaps root growth begins in fall so that roots are in place and ready to do their job as soon as tops poke above the ground. For the same reasons, for those covered in Robert’s article, I’ve always planted garlic as early as possible. Some falls some green does show above ground but the bulb does not suffer any cold damage.
Great article, full of information and explanation! Thank you!