Four Tips for Your Best Late Summer Greenhouse Harvest

By Trevor Dueck of BC Greenhouse Builders

Late August is a bittersweet time for gardeners. The outdoor garden may never have looked better, but you can feel the season winding down, and most growers find themselves wishing for just one more month on either end of it.

That wish is exactly what a greenhouse delivers. Even in Northern climates, a greenhouse grower can plant in April instead of late May, and instead of rushing to harvest tomatoes in a hurry before the first September frost, they can pluck them from healthy vines well into October. Season extension isn't the only benefit of greenhouse gardening, but that extra month of spring and summer on each end does wonders for a harvest.

Whether your greenhouse is already working hard or still on your wish list, here are four tips to make late summer the best stretch of your growing year.

Fertilize Carefully

By September 1, it's too late to "push" lush growth, so set aside your regular high nitrogen summer fertilizer and switch to one packed with micronutrients and very little nitrogen. To check how much nitrogen a fertilizer has, look at the first number on the label. A 20-20-20 blend is 20 percent nitrogen, while switching to something like 2-8-4 means you're only adding 2 percent.

The numbers on fertilizer packages vary greatly, but if nitrogen is too high late in the season, it stimulates new growth and lots of green leaves at exactly the wrong time. Read the fine print at your garden center, because that's probably the only way you'll find a low nitrogen fertilizer today. Look for one with the added benefits of kelp and micronutrients as well. Micronutrients such as magnesium, iron, and calcium build plant strength without adding to lush growth, and kelp makes the fertilizer more available to plants.

Keep the Fan On

Air circulation becomes especially important as the evenings cool. Moving air stops moisture from settling on leaves and causing fungus growth when temperatures dip in late summer. A small circulation fan will do the job in a compact 8x12 greenhouse, while a larger horizontal airflow fan suits a 16x20 space.

This tip is easy to overlook, but fall is a key time to keep the air moving. If possible, keep the screen doors on your greenhouse open well into fall too.

Start New Plants

Late summer is prime time to sow fresh lettuce and arugula, and in the warmth of a greenhouse, seeds can germinate within a day or two. If your family eats salad year-round, this second sowing keeps the supply going just as the first planting runs low. It also replaces heads that have become overgrown and elongated in the heat of the summer, since bolted lettuce loses its flavor once it gets big.

A fall harvest in the greenhouse means tender, tasty greens on the table long after outdoor lettuce has called it a season.

Keep Shading

Conditions inside a greenhouse are perfect for growing into fall, but as the sun drops lower on the horizon, it beats down hard on south facing beds. That's when shade cloth earns its keep.

For quick, targeted shade, push lengths of #9 wire into the ground about 2 feet (60 cm) apart and clip aluminet shade cloth over the wire. Outdoors, this keeps lettuce crops from getting too much light. Inside the greenhouse, the same trick works as a temporary cover on newly planted lettuce along the south facing side, keeping it soft and tender longer.

The larger shade cloth covering your greenhouse will come down by late September, but it only covers the top. The hot spots, like those south facing beds, benefit from this bit of extra shade.

One More Month, on Both Ends

A late summer greenhouse rewards steady attention: gentle feeding, moving air, fresh sowings, and smart shade. Put those four together and the harvest doesn't wind down in September. It simply keeps going.

BC Greenhouse Builders has spent 75 years building greenhouses that stretch the growing season for gardeners in every climate, from mild coastal backyards to the far north. To start planning yours, visit bcgreenhouses.com.

All articles are copyrighted and remain the property of the author.

Article URL:
https://www.gardensmart.com/?p=articles&title=Four_Tips_for_Your_Best_Late_Summer_Greenhouse_Harvest