Compost in Seed Starting Mix: Recipe for Success….or Failure?

A recent question posted to the Garden Professors blog Facebook group (a place where you can join and join in conversation of garden science) asked about the potential for compost added to seed starting media to cause failure in germination.  It is a good question, and one that seems to have several different camps – from garden hero author folks swearing by it in their (non-peer reviewed) books, to fact sheets saying it isn’t a good idea.

I’ve always promoted that the best practice for seeds starting is using a sterile media to avoid such problems as damping off.  Many of the problems I’ve heard associated with compost and seed starting are that improperly finished compost can introduce disease microorganisms to the media or cause phytotoxicity, it can make the mix too heavy and thus create anaerobic conditions that starve emerging seedlings of oxygen or cause decomposition, and there is the potential for residues of herbicides in composts using farm waste, manure, or lawn clippings as a feedstock. But does compost really pose a risk to seed starting?  I decided to take a very quick spin through the literature to weigh the possibilities.  Here are some of the potential issues and what a quick glance at the literature says.

Keeping the Germs out of Germination

Compost, even finished compost, has a high microbial activity.  For the most part, the fungi and bacteria in compost are good guys that pose no threats to plants, they decomposers or neutral.  But incorrectly managed compost can also harbor fungi such as Pythium and Rhizoctonia that cause damping off or even other diseases such as early and late blight if diseased plants were added to the compost and sufficient heat levels weren’t maintained.  Composts that don’t reach 140°F and maintain that temperature for several days to kill off potential pathogens run the risk of introducing diseases into seedlings.

Many promote the use of compost and compost products for potential antagonistic effects on bad bacteria.  We’ve discussed compost tea and the lack of conclusive evidence that it has any effect on reducing disease here many times before, and this article found that there is no significant effect of compost tea on damping off.  Some other articles, such as this one, did find that commercially prepared composts added to media did suppress damping off.  However, it is to be noted that these are commercially prepared composts, which have a strict temperature requirement and often require testing for pathogen and bacterial populations.  Many home composters aren’t as proficient at maintaining temperatures suitable for pathogen elimination.

Even if the compost is pathogen free, introduction into a germination media could potentially increase the population of pathogens already present in the media (or that land on it from the air) by providing a source of food for bacterial and fungal growth.  The sterile mixes aren’t just sterile from a microorganism perspective, they’re also sterile from a nutrient perspective as well to help inhibit potential pathogen growth.  The seeds come with their own food, so it isn’t needed for initial germination – the seedlings should be moved to a more fertile mix once they’ve established their first set of true leaves.

Image result for damping off
Damping off, source

You may be saying- “but we also direct sow seeds outdoors, where there’s lots of pathogens present in the soil.”  While this may be the case, damping off is still a definite problem in direct sowing and the loss of investment in materials, lights, and time is generally much lower (and less painful) than in indoor seedling production.  This is especially the case for large operations or for home gardeners who grow lots of stuff from seed.

This is the main issue that leads to the best practice recommendation to use a sterile seed-starting mix that doesn’t contain compost.  If a mix contains compost, it should be from a commercial enterprise that follows best practices or  pasteurized.

Maturity isn’t just for wines, cheeses, and people

Continuing to talk about proper composting, improperly finished compost that hasn’t properly matured (finished composting) can also lead to problems with seed germination.  Unfinished compost can still have woody material included, which has a high C/N ratio and also contain/release phytotoxic compounds during the decomposition process. The presence of decomposition microorganisms in a high C/N ratio means that there is still decomposition happening, which requires nitrogen for the process.  With absence of nitrogen in the media, the nitrogen from the seed or the seedling can be leeched out, effectively causing mortality after or even before germination.  The tender seedling serves as a source of N for the decomposing fungi.

We’ve had this discussion before when it comes mulch.  While mulch is perfectly fine on top of the soil, if it gets mixed into the soil there could be potential implications on N availability.

A germination bioassay is one tool commonly used to test for compost maturity.  Quickly germinating (and inexpensive) seeds are germinated on the compost (or on filter paper soaked with an extract from the compost in some commercial operations).  The rate of germination vs germination failure can give some insight into the maturity of the compost.  This paper discusses the use of the technique for commercial sawdust compost used for potting media.

You can use a bioassay of your own to test for compost maturity (or herbicide persistence, discussed later) for applications in your garden.  Sow an equal number of inexpensive, fast-germinating seeds like radish or lettuce sown on the compost with a control sown on moist paper towel in a bag.  Compare the number of germinated seeds and thriving seedlings after several days to see if there is an issue with the compost.

Keeping Things Light

One other quality required for seed starting media is a good level of porosity (pore spaces) for the media to hold air.  Air (oxygen) is important as it is needed by the roots for respiration.  If the media is too heavy or holds too much water you run the risk of hypoxia, or lack of oxygen, in the roots.  This can result in root die off and subsequent seedling failure.  Most seed starting media are composed of very light materials such as peat moss, coir, vermiculite, or perlite for this very reason.  Compost, by nature, is a more dense material with less porosity and has a higher water holding capacity.  Therefore incorporation of too much compost can create the potential risk of compaction or excessive water holding in the mix.

When Persistence Doesn’t Pay Off

Most herbicides break down during the composting process through a variety of physical and biological interactions.  However there have been reports of some herbicides that are persistent after the composting process, resulting in a residue that could damage plants grown using the compost (see this paper for some examples).  Many of the reports show the damage manifesting in mostly large applications of compost to gardens.  However, the more fragile nature of germinating seeds and young seedlings make them especially susceptible to herbicide residue damage.  For further discussion (and examples of bioassays used to detect herbicide residues), check out this paper.

So the potential for pathogens, risk of improperly matured compost, effect on porosity, and potential for herbicide persistence present some significant risks to germination if they are incorporated into seed starting media.  These are the risks that cause many sources to promote using sterile seeds starting media, and I think the advice is well founded.  While some may not experience these possible issues, the potential is still there.

Starting Seeds with Success: Best Practices

As we edge closer to spring it is time to start getting ready for the active growing season.   Many gardeners kick off their gardening year early with indoor seed starting to prepare for the upcoming season.

Starting your own seeds is an excellent, and often economical way to prepare for your year of gardening. Whether you grow vegetables or flowers (or both), starting from seeds can offer many benefits. Of course, there are some dos and don’ts for getting the most mileage from your seed starting endeavors.

I recently connected with Joe Lamp’l, host of the Growing a Greener World show on public television and the more recent The Joe Gardener Show podcast to talk about advanced seed starting techniques and technology.

You can follow the link below to listen to the show on your computer, or find it on Stitcher or iTunes (links included on the show page, too).  In addition to the podcast, the show page features extension notes on everything we chatted about with links to good reading materials.

Seed Starting Indoors: The Joe Gardener Show featuring GP John Porter

Here are a few of my best seed starting tips:

  • Be economical. One of the great benefits of starting plants from seeds is saving money. A packet of several (even hundreds) of seeds is often around the same price you’ll pay for one plant at the garden center. Of course, if you go out and splurge on the fancy (and expensive) seed-starting systems you see in your garden store or favorite catalog you may end up investing more than you planned. Instead of fancy seed starting trays or peat pellets and pots, use low-cost or recycled items such as takeout containers or shallow disposable aluminum baking pans to start your plants.  Remember that if you are reusing containers, especially ones that have had plants grown in them before, that sterilization is key in reducing disease.  Thoroughly wash the containers, then dip in a solution of 10% household bleach (1 part bleach : 9 parts water) to disinfect.  There are some horticultural disinfectants out there, but bleach is usually the easiest for home gardeners to get since you can pick it up at the local store.
  • Start seeds in clean, sterile seed-starting mix. This is one area where I don’t skimp. You’ll want to use a sterile mix that is primarily made of peat or coconut coir. It is lightweight and pathogen free and also low in fertility, so you will be less likely to lose plants to such issues as damping off (a fungus that rots the seedlings off at the base). Using regular potting mix may work, but increases your chances of such issues. Plus, seeds are equipped with enough nutrients to make it to their first set of true leaves before they need anything from the soil. I know that some sources say to use mixes with compost in them, but unless you know 100% that the compost got hot enough to kill all pathogens (140 degrees plus for several days) you could be introducing diseases to your plants that could affect them in the seedling stage or in the future.
  • Once the seedling has its first set of true leaves (the second leaves that appear), you should transfer it to an individual container/cell/pot with regular potting soil. At this point, the plant will need to have nutrients from the soil to grow healthy. You’ll want to loosen the plant from the seedling mix (I use a chopstick) and lift it by the leaves (not the stem). Temperature control is key.
  • Heat is usually the most important factor in coaxing your seeds to germinate, so placing your newly sown seeds in a warm (around 75 degrees F) place will help them germinate faster. Fast germination is key for making sure you get the optimal number of seeds sprouting. However, moving the seedlings to a cooler place (around 65 degrees) after they’re germinated will make them grow sturdier and keep them from getting thin and leggy. Most people laugh when I tell them, but one great warm place to start seeds is on top of the refrigerator.
  • Light is necessary for good plant growth. Most seeds don’t require light until they get their first true leaves, but after that you’ll want light to keep your plant healthy. Some people are lucky to have a good, sunny (usually south facing) window with plenty of light. Otherwise you’ll need to invest in some lighting. The most economical option is a basic shop light fixture from the hardware store. You can buy plant lights, or full spectrum lamps for it, but if they prove too difficult (or expensive) to find, use a regular warm fluorescent and cool fluorescent bulb to get the right light spectra. You’ll want light on for about 16 hours per day. If you are using a window, be sure to turn the plants regularly to keep them from
    Image result for led plant lights
    Blue and Red LEDs Source: Wikimedia Commons

    growing in one direction.  As LED lights become less expensive, many home gardeners are checking them out for home seed starting.  You can use a full spectrum white LED bank, but plants primarily use red and blue light so you can also find high-intensity LED banks for plant production that are blue and red (makes purple!).  Some research is emerging that a tiny bit of green light helps growth, so some newer systems are incorporating a touch of green, too.

  • Don’t get started too early.  Look at the packet for the number of days/weeks before last frost to start your seeds.  If you start them too early, you could end up with spindly, leggy plants or ones that have grown too large for their containers. Even if you have good lighting, your plants will not thrive being cooped up in the house too long.
  • What about fertilizer? Up until the first set of true leaves, seedlings don’t need much in the way of fertility.  When they’re put in larger containers or cells, a good potting mix (usually containing some type of fertilizer or nutrients) will get you most everything you need….to a point.  If you’re growing in small containers, say those cell packs where you have very limited soil, you may find that you need to provide supplemental fertility after a few weeks.  There’s only so many nutrients in that potting mix in small amounts, so if you are holding your plants for longer than, say, six weeks you may need to apply a water-soluble fertilizer or start off with a slow-release fertilizer.  Larger containers, say a 3 or 4 inch pot, may have enough soil to have sufficient nutrients to get you to the point of transplanting.

Creative Lighting for Seed Starting

As we get close to the time to start tomato, pepper, and other seedlings indoors, I thought I’d share this picture of my older sister’s seed starting setup from few years ago:

lamps et al

Two desk lamps with compact florescent bulbs. Not traditional, but worked great. Just a reminder that you can get creative when it comes to lighting for seedlings, using whatever fixtures and layout works for your space. The only rules are to use florescent or LED bulbs, not those old fashioned incandescent bulbs which have poor light for plants, and err on the side of more light rather than less to make sure you get compact, healthy plants that will transition to the sunny outside world without drama.
Joseph Tychonievich


Why you (probably) shouldn’t be starting seeds yet

As a beginning gardener I learned that to give plants like tomatoes and peppers more time to grow and produce the largest possible crop, it was best to start the seeds early indoors.

gazaniaseedlingsAs soon as I learned that, I wondered: Well, if starting my tomatoes 6-8 weeks before transplanting them outside is good, surely 10 weeks would be better, right? Or 12? Or 16?

Turns out, earlier isn’t always better, and here are some of the reasons why.

First, you probably don’t have enough light. If, like most home gardeners, you are starting seeds under florescent bulbs, it is difficult to give sun lovers like tomatoes and peppers enough light. Light intensity drops off rapidly as you move away from the bulbs, so you know to keep the bulbs right above your seedlings. This works great when the plants are small, but as they grow it becomes very difficult to give both the tops and the bottoms of the seedlings enough light. The result is dying lower leaves and spindly, unhealthy growth.

rootboundSecondly, you are almost certainly going to get some crappy root systems. If you’ve followed this blog for a while, you’ve no doubt read Bert and Linda talking about all the potential problems with the root systems of container grown trees and shrubs. Well, most of the same problems develop with other plants grown in small containers. The roots start circling and they are slow to grow out of the rich soil of the container and into the native soil around them once transplanted into the garden. The longer your transplants grow indoors, the more likely they are to develop problematic root systems. Keeping transplanting them up to larger and larger containers can help mitigate the problem, but that quickly takes up far more space than most home gardeners have for there seedlings.

How big and impact that circling root system will have on the health of the plant varies by species. My personal experience growing zinnias, for example, is that they handle circling, pot-bound roots so poorly that plants from seeds sown directly in the garden quickly over-take and out-perform plants started weeks earlier indoors.

So follow the recommendations for the timing of seed starting. It really does work better. You should be able to get advice on when to start seeds from the catalogs you are shopping, extension offices, or you can use Margaret Roach’s excellent seed sowing calculator.

If you DO decide that earlier is better, that you can provide the light and generous pot sizes to avoid problems, there’s no harm in giving it a shot. But if you do, try starting a second batch at the later, recommended, time and growing the two side-by-side in the garden so you can really compare and see which perform best in the actual conditions of your garden, and if all that extra time and space under your lights or in your greenhouse was really worth it.

Joseph Tychonievich

Cold Stratification for seeds

You may think of spring as the time for seed sowing, but I do a lot of seed planting now, in the fall. The reason is that most of the cold hardy perennials, trees and shrubs that I like to grow produce seeds that require cold stratification.
This simply means that they require a period exposed to cold temperatures while the seeds are moist and hydrated before they will germinate.

The requirement for cold is a pretty straight-forward adaptation to life in cold climates. Seeds that ripen in the summer and fall might not have time to get established before winter if they germinated right away. The requirement for cold means the seeds don’t actually sprout until spring, giving them a full growing season to get ready for the next winter.

An outdoor seed bed is an easy way to stratify seeds

You can — and I used to — give seeds this cold period in the refrigerator. Three months in the fridge in a plastic bag with a damp paper towel to keep the seeds moist is long enough for most everything, though the exact period of cold required varies by the plant. The fridge works, but I think it is way easier to just do it outside. So the past few days I’ve been busy sowing seeds out in my outdoor seed beds. They’re just raised beds, filled with potting media, and covered with a screen lid to limit the number of weed seeds that blow in and keep disruptive animals out. I plant my seeds in the fall. Come spring, after the seeds have had their dose of cold, they sprout.

Dianthus seedlings ready for transplanting
Dianthus seedlings ready for transplanting

Once the seedlings have grown on a while, I dig them out, separate the individual plants, and put them out into their final locations in the garden.

Individual seedlings separated and ready for planting
Individual seedlings separated and ready for planting

I do this with more and more seeds every year, even for perennials that don’t require a cold period to germinate, simply because it is so easy. No fussing around with lights or checking the calendar or even much watering. Just plunk the seeds in, and dig out the plants once they are big enough.

Cactus grafting fun

I’ve been grafting cactus this summer, and made this:

grafted cactus

It is a seedling of the gorgeous hardy cactus Echinocereus reichenbachii, grafted onto Pereskiopsis spathulata, an odd, leafy cactus I wrote about earlier.

Why do this? Other than the fact that it is darn cool? Well, because that vigorous, fast growing rootstock pumps a lot of energy into the cactus grafted on top, making the grafted cactus grow a LOT faster than left on its own roots.

grafted cactus startThis is a (terrible, blurry) picture what the graft looked like when I first made it back in July. Just three months later it has grown to enormously, while the seedlings I left on their own roots look pretty much the same. I’ll let it grow on the graft for a while, then probably next year some time, cut it off, and move it into the garden, getting me to a reasonably sized plant in a reasonable amount of time.

So… if you want to speed up the growth of a pokey cactus, try grafting it. The process is crazy easy, lots of fun, and very thoroughly explained here.