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Warm Season Direct Seeding in High Soil Temperatures

This is the time of year when you hear gardeners saying things like: “Have you put your garden in yet ?” or “Put the garden in this weekend!”. These comments reflect the notion that vegetable gardening and producing food is a singular one-time event that occurs sometime during the month of May. However, this is far from the truth. In fact, when you have a well thought out plan in place (at any scale: backyard garden to small farm), you could be planting something during every month of the calendar year. Of course here in Cook County (hardiness zone 5b and 6a), that requires season extension techniques as well as succession planting. Both of which are detailed topics that will be discussed another time. For now let’s focus the discussion on a timely topic that is often a challenge for gardeners and growers: Getting seeds to germinate in the soil during the warm summer months.

Soil Temperature

The temperature of the soil or growing medium is the single most important factor when it comes to seed germination. Everyone has struggled with the often fickle spinach seed! Although 70F is often cited as the optimal soil temperature for spinach germination, you may only get 50-52% normal seedlings at that temperature. Whereas, between 50 and 60F the percentage of normal spinach seedlings is around 82-91%, with the higher percentage of germination in the low 50F range. Yes, the spinach seed might germinate a little quicker at 70F (6 vs. 12 days), but the emergence may be a bit more erratic as the soil temperatures climb higher. Purchase a decent soil thermometer and take readings at a depth of 2-4” to monitor your soil temperature.  In addition, consult the numerous resources out there on soil temperature and vegetable seed germination. Here are few to consider:

“The New Seed Starter’s Handbook”

“Knott’s Vegetable Growers Handbook”

Seed Storage

Storing seed in a refrigerator within enclosed containers will do more than extend the viable life of your seed. Certain seeds can be tricked into thinking they are going through an extra “winter”. Lettuce and spinach are both good examples of this. Putting the seeds in a double zip locked bag for 1-2 weeks prior to planting out in warm summer conditions can aid germination.

Reducing Outdoor Soil Temperatures

Depending on the scale of production, there are some cultural methods you can employ to physically reduce the temperature of the soil.  Using cool ground water to irrigate with is one option. Another is to actually apply ice to the beds themselves. Of course this is only practical at certain small scales, but is an effective way to cool the soil and water it at the same time. Waiting to plant at the end of the day and into the evening hours is another technique to trick the seed into germination, especially with quick germinating seeds. Utilizing 30-50% shade cloth over low tunnels is a structural technique that is often used to cool crops themselves during the warm summer months. However, this same fabric can also be effective in reducing the soil temperature by up to 10F.

Sow Indoors First

Sometimes more controlled conditions can be found inside an automated greenhouse or your germination bench in a basement. Starting seedlings under more favorable conditions inside, opposed to planting them outside in hot soils, may be a method for overcoming erratic germination with certain crops. You may also have some room in a refrigerator dedicated to seed storage for a flat of lettuce to germinate. It will be important for this space to be kept clean and dry if you have other things in the fridge. Putting the flat in a plastic bag will help accomplish this. Just remember to open it up every day to get oxygen to the flats.  Basement floors are usually cooler than shelving areas. Using any microclimate at your disposal could be useful for germinating seeds. Starting salad green crops in seedling flats/trays/soil blocks or in multiplant plugs(multiple seeds per cell not thinned), then transplanting those plugs out into the garden/bed in a close spacing is another technique. This could substitute for a normal direct seeding of a crop when soil temperatures are still too high outdoors. Be sure to transplant on a cloudy day or late in the day, and water the plugs in well to avoid shock and wilting. Plugs will need to be well watered until they are established.

Other Techniques

There are a few other cultural techniques to consider when enhancing summer germination. Light, is less of a factor, with the exception of lettuce and celery. Both of those vegetables germinate better when given a little bit of light, especially at high soil temperatures. So be sure to cover these seeds lightly or not at all.  Getting enough water to germinating seeds in high temperatures is often a challenging task, in particular for soils that are prone to crusting, which makes emergence for small seeds very difficult.  To avoid this, do not over-cultivate soils before planting.  You can also water a day ahead of planting in order to reduce the amount of water needed the day of planting. Avoid rolling seed beds or tamping the soil surface after planting. Very light mulching with grass clippings or sand may also help avoid undesirable crusting of the soil. After planting, watering should be shallow and often to promote the quickest germination for summer planted crops. Drip irrigation is an effective watering strategy for established crops, but can often be inconsistent on a germinating seed bed unless the drip tape has very close emitter spacing (< 12”) and multiple lines are used.

Soaking and Pre-Sprouting Seeds

Legumes, like beans and peas, can benefit from soaking overnight before planting in the field. If a seeder is being used, letting the seeds dry a little bit before planting will avoid planter clog up. It is important to not let the seed coat split before planting out as to avoid potential fungal issues

Pre-sprouting seed right up until the seed has just germinated is another way to get seeds going before setting them out in warm conditions. Letting the first shoot grow to no more than 0.2” is ideal for most crops. The process involves first soaking the seeds to allow them to imbibe the water needed for germination, draining off the excess water, then rinsing twice a day over a screen until germination. Store them in the refrigerator until the first shoot barely emerges to the 0.2” length. One novel way of planting pre-sprouting seeds is to sow them in a homemade paste. This could be made out of a starch or cellulose base. Cornstarch, gelatin, pectin, agar are all examples of “planting paste” base materials that could be used to plant the pre-sprouted seeds in a furrow in a garden bed. Look online for specific recipes. After the pre-sprouted seeds are added to the paste and put into a plastic bag. The bag is then cut at one corner to squeeze out the desired rate of about 1 tablespoon of paste per linear foot. This will take some experimenting to get the desired number of seeds out per linear foot. Of course this technique is impractical on a large scale, but on the small scale could be a technique employed to help along slow germinating seeds.


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