In years gone by it was common to clean up the garden in fall. It does result in a neat clean looking garden, but then we learned that all kinds of beneficial insects overwinter in that messy looking stuff on the ground, so we started to do cleanup in spring instead. To understand why, read this; Spring Cleanup Advice – Do it Right .
Now things have changed again. We are being told not to cleanup until temperatures are consistently above 10 C (50 F) to allow insects time to leave their winter protection. If you look at the recently circulated meme below, you have to wonder if this recommendation is valid. Does it really have to be 10 C? I see all kinds of insects when there is still snow on the ground. And what does “consistently ” mean? Is that nighttime or daytime temperature? Is it a daily high or low? How much harm will you do if it is only 9 C (48 F)? Is there any real science that supports this idea? Let’s find out.
Analyze The Meme
When this gets posted on social media it quickly splits people into two camps. A number of people post to support the idea and comment on how important it is. A different group are skeptics and don’t believe it, or maybe they just want to keep doing what they’re doing. What surprises me is that only a very small number of people question the actual contents of the meme.
- What does “consistently” mean?
- How critical is the 10 C (50 F)?
- How long does the temperature have have to stay above this critical value?
- What does “cleanup” mean? Is there a way to cleanup that does not harm the insects?
If you have been on the internet for more than a year, you probably know that most of these memes mean well, they may contain an ounce of reality but they are rarely correct. Everyone should be questioning them. Until you can answer the above questions, you really don’t know if you should follow this advice. In fact, you don’t even know how to follow it.
Spring Garden Cleanup At 10 C
The above meme is not the only one making this suggestion and some websites also promote this idea. On Savvy Gardening they suggest not cleaning the garden until ” the daytime temperatures are consistently above 10 C (50 F) for at least 7 consecutive days”. This is more precise, but not precise enough. If the temperature hits 10 C for 15 minutes and then drops below 10, does it count as a positive day? Is that consistent enough?
Illinois Extension suggest we shouldn’t clean up until “temperatures are consistently over 50 F degrees overnight.” In zone 5 that doesn’t happen until summer.
A big problem is that nobody seems to define the target temperature well enough so that gardeners could follow the advice.
How is “Cleanup” Defined?
I could cut everything back, rake it all up and put it in the recycle bin or the yard waste bin. Or I could collect everything and pile it up so insects can still escape from the pile. Or I could use my cut and drop method; cut everything back but leave it right in the garden. They are all “cleanup methods” but surely the first is more destructive than the last? The cleanup method used is important and must be defined to understand the meme.
What Does The Science Say?
I have not been able to find any research that has determined that 10 C is the right temperature. Dr. Langellotto also looked for some scientific evidence and found none. She thought it might have come from a bat study that dealt more with aquatic flies than pollinators.
The meme does not seem to be based on any specific scientific data but we do have some science that supports the general idea of a late spring cleanup.
Insects Are Cold Blooded
Insects are cold blooded. When it is too cold they become inactive. As temperatures rise they become more active. Some early flowering plants like the skunk cabbage, even generate heat in their flowers to attract cold insects. The other thing we know is that each insect species has its own activation temperature.
Spring Bee Activity
In Minnesota most stem-nesting bees have emerged by mid-June, but they emerge over a long period of time. Mason bees are some of the first to emerge.
In Portland, Oregon, the small carpenter bee, Ceratina acantha accounts for 10% of urban bees and it nests in pithy stems. They emerge in April in this zone 8/9 climate, so cleaning up in May won’t harm them. In zone 5 you would have to wait until June.
The flight time of various bees in Portland have been measured. If we look at bumblebees for example, they start flying as early as February and as late as April, depending on the species. These bees are ground nesters, but the organic layer on the soil is still important to them.
Gardens have fewer spring-flying bees and fewer soil nesters compared to non-garden sites. There are several potential explanations for this pattern including less availability of spring-blooming trees and other plants in gardens, and a lack of nest site availability for soil-nesting bees. Garden practices are clearly affecting bee diversity.
A study done in Ontario looking at emergence of cavity nesting bees and wasps in a lab setting and found a 30 day difference between early and late emergence.
Spring Butterfly Activity
Some butterflies overwinter in leaf piles and can’t really fly until the temperature stay above 15 C (60 F), but there have been few studies. The vast majority that overwinter in gardens tend to be the non-native cabbage white butterfly, Pieris rapae, which overwinters in bark and crevices above the soil. They would be unaffected by garden cleanup.
Control Of Pest Insects
I know gardeners don’t want to protect pests, but some pest insects have been studied much better than pollinators. Insect behavior does not change just because we label them pests.
The common advice in Utah is to spray May 1 to catch the codling moth on apple trees just as the caterpillars hatch. Their historical hatch dates were May 15 in 2005, May 5 in 2006, and April
30 in 2007. Clearly picking a date does not work very well – it is better watching the weather. Picking a date for spring cleanup does not work any better.
A better approach is to use degree days, also called growing degree days. Insects need to accumulate a certain amount of heat to reach certain stages of life such as egg hatching or adult flight. Degree days is a way of measuring accumulated heat. To see examples of calculating degree days in Fahrenheit click here, and in Centigrade click here.
The key point for our discussion is that each insect has a different threshold value for degree days. When the daily average temperature is below the threshold, it does not add enough heat to benefit the insect. Heat is only valuable once the average temperature is above the threshold. This threshold is known for some common pests, but it has not been established for most insects. In that case a general value of 10 C or 50 F is used.
Each insect requires a certain number of degree days in order to change its activity: leave the nest, hatch from an egg, hatch from a pupae or start active flight. Degree days are well defined and easy to measure.
Three supper warm days in a row are just as good as twice that number of cooler days provided they are above the threshold. The important part is the total amount of heat, not the number of days.
Temperature In The Meme
You might have noticed that the above general threshold for degree days matches the temperature in the meme. I think this is where the temperature came from.
The meme’s author misinterpreted the general threshold for degree days and turned it into an actual minimum temperature and added the term “consistently above”.
When Should A Gardener Cleanup In Spring
Since the meme is incorrect and not clear enough to be followed, we need different advice. We do know a few things.
- Insects emerge from their winter hiding places over a wide range of temperatures and over a several month period.
- The longer you can wait for cleanup, the better it is for all insects.
- It is not realistic for most gardeners to wait until all insects have emerged. It will be summer before that happens.
Perhaps more important than the temperature is how you clean up the garden. Raking up the organic litter will harm some insects. It is better to leave it on the ground unless it is too thick for plants to emerge and then only remove the amount that needs to be removed.
When you remove organic litter try to pile it up in an out of the way place, using low piles so insects can easily crawl out. The higher the pile is, the more insects you will trap. If you compost, hold off on using this material until mid summer.
Some of the slowest emerging insects are cavity nesters and they like nesting in the old stems that are still standing up in spring. Leaving them too long is not a good idea because perennials will start to grow at their base. Instead, use the cut and drop method. Cut the stems into 6″ lengths and drop them on the ground so that cavity nesters can still escape from them.
Some garden cleanup will not harm insects. Pruning trees is unlikely to affect insects and can be done very early. Cutting back ornamental grasses can also be done at any time because insects don’t nest in the thinner stems. In fact grasses should be cut back early. If you wait too long, birds nest in them and then you disturb the nests.
Ground nesters are less disturbed by cleaning up since they are protected under ground. However, applying mulch at any time can cover up new nests and prevent them from escaping. Also, many ground nesters won’t make nests in mulch. Mulching is better for plant growth, prevents weeds and provides a hiding place for some insects. Large chunky mulch is better than finely shaved mulch because it packs down less but every kind of wood mulch is eventually knitted together by fungi growth.
Dealing With A Messy Garden
Why do we clean up the garden in spring? Nature doesn’t. We like things to look neat. Cleaning up a garden has more to do with a perceived need and a fear of what neighbors might think. So get a new garden sign and do less cleaning.