Here are some tips on moisture management during this frigid winter weather.
PROBLEM: When major cold fronts hit during January and February, everything slows down. Not just shipments of unrooted cuttings (URCs) and young plants, but your crops on the bench, too! On a macro-level, temperature controls the rate of plants’ development, but these cold snaps do more than just slow leaf unfolding rate. A major short-term impact that sudden drops in your greenhouse air temperature have is the rate at which plants move water.
NICK’S TIP: One of the biggest mistakes a grower can make when greenhouse temperatures suddenly become “lower than ideal” is not adjust moisture management. Whether you have plugs, freshly stuck URCs or finished crops on the bench right now, dial back and dial in your moisture management strategy to keep plants healthy and on track until temps return to normal.
Plug Moisture Adjustments
Soil moisture in freshly sown plug trays should be dialed in as they leave the sowing line, regardless of whether they're destined to enter a germination chamber or be bench-germinated. However, once they’re on the bench, you’ll need to adjust how trays are watered (or moistened, in the case of bench-germinated crops).
• Moisture-sensitive genera like gerbera and annual vinca (catharanthus) need to dry down quickly once they complete Stage I (radicle emergence). That said, it can be a challenge to get soil moisture from a level 4 to 5 (on the 1 to 5 scale; 1 = air dry, 5 = saturated) down to a level 2 or 3 when greenhouse temps are low and vents are buttoned up tightly. Do what you can to hasten dry-down in other ways. Gap your plug trays slightly to increase air flow, keep HAF fans running and lean on bottom heat as much as you can to hit that dry-down target within less than about 48 hours.
• For plugs in Stages 2, 3 and 4 (cotyledon emergence, first true leaf emergence and bulking/toning, respectively), employ the same strategies to maintain wet-to-dry cycles in about 48 hours or less as much as possible. This will help to reduce hypocotyl (Stage 2 plugs) and stem stretch (Stage 3/4). Also, reduce water/fertilizer application volume when you need to irrigate or fertigate. For example, if you normally water up to level 4 and dry down to level 2, only water up to a level 3 instead. A slight reduction in water applied won’t stress the young plants, but it WILL allow you to maintain appropriate wet-to-dry intervals in your plugs.
• Whether you irrigate plugs via boom or by hand, decrease the flow rate or size of your water streams to achieve better control of application volume. For example, if you hand water, switch to a breaker with a greater number of smaller holes and dial your hose pressure back to achieve better control of application volume with each pass. Similarly, if you boom irrigate, temporarily switch to boom tips with a lower flow rate and run the same number of passes that you would under normal growing conditions. Re-assess the moisture level in your trays after a few minutes and make another pass or two if necessary.
Liner Moisture Management
Excess free moisture on your URCs is bad enough, but surplus moisture when greenhouse temps are suboptimal is an even bigger problem for liners. Take some extra time and fine-tune your mist settings to ensure leaves and stems aren’t staying overly wet and that excess moisture isn't accumulating in your liner media.
• Check liner media moisture and add a small amount of moisture to your mix before URCs are stuck. If media is too dry going into the tray, you'll need to water cuttings in heavily to get everything uniformly moistened after stick. This is the last thing you want when greenhouse temps are cooler than ideal because cool temps and high moisture are conditions under which Botrytis thrives.
• When you adjust mist settings to deliver less moisture, dial back the mist duration (runtime at each event) rather than the frequency (runs per hour). Continue to reduce the runtime until the leaf surface is about 95% dry before the next mist interval hits. This will help to minimize drought stress while reducing the likelihood of moisture accumulation in the liner media.
• If possible, use a VPD (vapor pressure deficit)-driven system to manage mist in your liner propagation area. These systems automatically account for the lower-than-ideal temperature and greatly reduce the number of manual adjustments you need to make as the grower.
• Long-term, consider implementing a fog system rather than a traditional mist system. These allow you to maintain very high relative humidity in your propagation area while virtually eliminating excess surface moisture or addition of unwanted water to your liner media.
Finished Crop Adjustments
Since finished crops have a much larger proportional soil volume, compared to young plants, this is where things can really go off the rails when greenhouse temps drop suddenly. Larger volumes of wet soil take a lot longer to warm up and dry down, so pay special attention to your finished crops over the next few weeks.
• As suggested with plugs, increase spacing between flats/pots wherever you can to increase air flow between plants. Keep air moving with HAF fans, and if you're in a position where you have to pick between different heating strategies to keep costs down, lean most heavily upon the system that can deliver the most radiant heat directly to the soil whenever possible.
• While I hope this seems like a no-brainer at this point, don’t water crops as heavily! Ensure that you pick up multiple flats/pots before you make the decision to irrigate or apply fertilizer.
• Resist the urge to make watering decisions based on the color of growing media at the surface of the container. For smaller containers, slide the pot/cell pack off and check the soil profile to see how far the soil column has dried down.
• Avoid wetting the floors as much as possible when you water. The more moisture you add to the surrounding greenhouse environment, the slower your crops will dry down. Turn off your watering wand between groups, keep the stream directly over containers and ensure boom tips shut off promptly at the end of a given row or section of crops that are being watered.
• In general, err on the side of being a little too dry at the end of the day than too wet. Cool, wet roots are more likely to succumb to root zone disease pressure than crops that are running more on the dry side!