The last weeks of marijuana bloom phase are crucial for the success of your entire season. Here are time-tested strategies for the finishing weeks of bloom phase so you get your biggest, tastiest, most potent harvests…

Incremental Marijuana Harvesting

We have an article that includes a detailed discussion about incremental harvesting here. Especially if you’re in love with a particular plant and thinking of regrowing it as a mother or as a rejuvenated plant to get a second harvest, you want to harvest a few buds a week before you anticipate your final harvest.

Dry and test the buds to know for sure how potent the strain is and how much you like it.

Flushing Marijuana

We have an entire article explaining why the common end-of-harvest flushing program is bad for marijuana plants, but mini-flushes are useful. Mini-flushing purges excess nutrients salts from the root zone and draws oxygen into the root zone.

We do mini-flushes using a product called Root, made by Rx Green. The product contains beneficial root microbes, sugars, and a material called formononetin. Formononetin promotes root growth. Every couple of weeks in bloom phase we feed only Root for one irrigation cycle.

Using this effective root zone microbes product during bloom phase increases root health and function in ways that increase production of cannabinoids and terpenoids.

Applying Ultraviolet Light to Cannabis

Some grow light companies include built-in or off-board ultraviolet LED chips or bulbs in their gear. That’s because some research shows that applying ultraviolet in the UV-A and or UV-B spectrum irritates plants, allegedly leading to plants producing more cannabinoids and terpenoids.

Research on using ultraviolet light, and far red light, is not yet conclusive, but some growers and grow light companies seem to think it is. They vigorously apply ultraviolet light during the last couple of lights-on hours starting in mid-peak bloom phase.

We’ve talked to the experts, and run crop tests. We’ve seen no conclusive evidence that ultraviolet light predictably boosts marijuana potency.

There is one guaranteed benefit from applying ultraviolet light in a specific way. Using the CleanLight Pro handheld ultraviolet light to apply 5-10 seconds of UV to plants just before lights go off in late bloom phase is very effective at killing powdery mildew and gray mold (botrytis) before they start.

Trimming & Supporting Bloom Phase Marijuana Plants

If you’ve topped and multi-branched your marijuana plants instead of allowing them to grow into their natural single-spike cola Christmas tree shape, you often find branches heavy with buds are bending away from your grow lights and may even break.

The least complicated intervention is the best. We start by tying plant tape around the mid-section of the plant and tightening it before we tie it off so all branches are supported, and oriented vertically.

This necessarily compresses some branches and buds close together, so examine the plant and cut away leaves and separate branches and buds to ensure air flow.

If one tie doesn’t work, we put one or two more up and down the height of the plant, girdling it in support, like a tomato cage done with plant tape.

We usually don’t need to use stakes, trellises or other supports, even for the fattest buds.

Trimming away large top and interior fan leaves increases light and air movement penetration deeper into the canopy, but remove as few leaves as possible, and never more than 5-10% of total leaves at any one trimming, and not more than once a week. It’s best to remove only leaves that show that their nutrients have been used up and they’re already dying.

Healthy leaves (including the largest leaves) are photosynthesis factories and nutrients storehouses for your bloom phase cannabis plants. If you remove more than a few leaves a week, you weaken your plants, causing them stress that can lead to hermies and loss of harvest weight and potency.

Monitor Marijuana Resin Glands

Bloom phase duration predictions provided by marijuana breeders are mere estimates and can be harmfully unreliable and inaccurate.

The best way to know when to harvest is by using powerful magnification to examine your plants’ resin glands.

Healthy, peak resin glands consist of a crystal-clear round globe on top of a stalk. In the last weeks of bloom phase for many marijuana strains, the resin glands turn cloudy and/or amber, indicating ripening.

Most growers harvest when more than 25% of their resin glands have gone from clear to not clear, but this tactic can misfire, because some strains naturally have cloudy or amber resin heads from very early in bloom phase.

A more reliable indicator, to use in conjunction with resin gland color and clarity, is whether resin glands are intact, vertical, firm, and structurally sound. When your marijuana plants are ready for harvest, 20% or more of your resin glands will have cloudy or amber heads, and stalks and/or heads will be falling apart.

Incremental harvesting gives you more nuance when it comes to precise harvest timing, regardless of resin gland condition.

Molasses for Marijuana

Contrary to what some fertilizer manufacturers say, adding sugars to your root zone doesn’t guarantee your buds will taste like sugar. Sugar isn’t absorbed by roots and transferred to buds.

On the other hand, if you have a healthy colony of beneficial microbes in your root zone, feeding a teaspoon of unsulphured organic blackstrap molasses per gallon of irrigation water during feeding once a week provides sugars consumed by beneficial microbes. Beneficial microbes work symbiotically with roots in ways that may boost bud taste and scent.

Note: use only organic unsulphured blackstrap molasses. Other types of molasses are inferior, if not harmful, when used in your marijuana growing.

Recording Marijuana Plant Details

When you’re close to harvest time, record details of your season and each plant’s performance. If you’ve been keeping good records, you can do a final analysis to determine bloom phase stretch, total height, bloom phase duration, scent, pest and disease issues, light intensity and other factoids.

These facts help you decide which plants you want to clone, grow again, or discard, and which marijuana strains match best with your grow room and cultivation style.

Monitoring for Cannabis Pests & Diseases

Daily close examination of leaves, stems and buds give you an early start on catching spider mites, thrips, whiteflies, aphids, gray mold, powdery mildew and other problems when they start. This makes it easier to stop the problems.

I study my leaves twice a day. On one recent morning, my leaves were clean. In the afternoon, I saw a few adult spider mites, but no eggs. The adults had just arrived, apparently. I sprayed with a rosemary-based deterrent. Because I caught the adults before they laid eggs, I didn’t have spray again. If I hadn’t caught the mites until a day or two later, they’d have already laid eggs, making it much harder for me to stop a full infestation.

Monitoring & Optimizing Vapor Pressure Deficit

Check out our article on vapor pressure deficit. Ignoring the relationship between plant growth, grow room temperature, and grow room humidity can create big problems in your grow op.

Especially in summer months when you’re using the air conditioner a lot, it’s easy for vapor pressure deficit (VPD) to go out of range because grow room air is too dry. Growers also have problems when grow op air is too humid.

It’s very difficult to keep VPD in the right range, which sometimes requires use of dehumidifiers and/or humidifiers. Take a look at the chart in the article, and you’ll have the perfect guide to work from.

We’ve just given you a set of actions to take during the last weeks of bloom phase. Some of these are appropriate during the entire season. When you use these tactics, you better ensure the health, potency, and harvest weight of every marijuana crop you grow.